fix(transcript): tlang en dernier recours + garde anti-blob

Le fallback &tlang= peut renvoyer la piste source en un seul bloc
de plusieurs Ko (throttle YouTube) : il passait devant les pistes
directes et son blob anglais etait mis en cache comme 'fr'.
tlang desormais etape 3 (apres directes signees), isBlobTranscript()
refuse les cues uniques > 2000 chars (fetch, yt-dlp et post-dedupe).
Tests 27/27.

server/index.mjs

@@ -18,7 +18,7 @@ import rumbleRouter from './rumble.mjs';
 import { providerRegistry, validateProviders } from './providers/registry.mjs';
 import { dedupeSuggestGroups } from './suggest.mjs';
 import { fetchWebSuggest, fetchOdyseeLighthouseSuggest } from './suggest-web.mjs';
-import { pickTrack, parseTrackText, parseVtt, dedupeTranscriptLines, orderedTracks, translatedFallbacks, firstPerLanguage, normalizeTranscriptProvider, transcriptTrackExt, looksLikeHtmlError, ensureFmtParam, isEmptyTimedTextBody, mergeTranscriptCandidates } from './transcript.mjs';
+import { pickTrack, parseTrackText, parseVtt, dedupeTranscriptLines, orderedTracks, translatedFallbacks, firstPerLanguage, normalizeTranscriptProvider, transcriptTrackExt, looksLikeHtmlError, ensureFmtParam, isEmptyTimedTextBody, isBlobTranscript, mergeTranscriptCandidates } from './transcript.mjs';
 import {
   getUserByUsername,
   getUserById,
@@ -3543,11 +3543,14 @@ r.get('/transcript/:provider/:videoId', transcriptLimiter, async (req, res) => {
       return res.json(empty);
     }
     // Stratégie de récupération (résultat des tests live 2026-09-28) :
-    // 1. Sonde rapide des URLs InnerTube (découverte, max 2, sans retry) :
+    // 1. Sonde rapide des URLs InnerTube directes (max 2, sans retry) :
     //    elles répondent désormais 200-vide depuis les egress filtrés.
     // 2. Dump yt-dlp -> URLs signées fraîches (`signature`/`expire`) fetchées
     //    en direct (max 3) : c'est la voie rapide qui fonctionne encore.
-    // 3. Dernier recours : `yt-dlp --write-sub` (client visionos + retries
+    // 3. Traduction serveur `&tlang=` (dernier recours avant yt-dlp) : sous
+    //    throttle, YouTube peut renvoyer la piste source en UN seul bloc au
+    //    lieu de la traduire — le garde anti-blob refuse ces réponses.
+    // 4. Dernier recours : `yt-dlp --write-sub` (client visionos + retries
     //    internes, le plus robuste : ~6 s, 80+ Ko vérifiés live).
     // On ne martèle jamais plus de 2 URLs mortes d'affilée : chaque 200-vide
     // ou 429 supplémentaire aggrave le throttle YouTube à la minute.
@@ -3560,7 +3563,7 @@ r.get('/transcript/:provider/:videoId', transcriptLimiter, async (req, res) => {
     const keepAllowed = (cands) => (cands || []).filter(c => allowedSet.has(transcriptPrimary(c.lang)));
     const markTransient = (msg) => {
       if (msg.includes(':429') || msg.includes('Sorry') || msg.includes('sorry')) sawRateLimit = true;
-      if (msg.includes(':empty_200') || msg.includes(':unparsable') || msg.includes('html_error')) sawEmpty = true;
+      if (msg.includes(':empty_200') || msg.includes(':unparsable') || msg.includes(':blob') || msg.includes('html_error')) sawEmpty = true;
     };
     const tryCandidates = async (cands, { retry429 = false, label = '' } = {}) => {
       for (const cand of cands) {
@@ -3568,9 +3571,12 @@ r.get('/transcript/:provider/:videoId', transcriptLimiter, async (req, res) => {
         for (;;) {
           try {
             const parsed = await fetchTimedTextLines(cand.track);
-            if (parsed && parsed.length) {
+            if (parsed && parsed.length && !isBlobTranscript(parsed)) {
               lines = parsed;
               workedLang = cand.lang || chosenLang;
+            } else if (isBlobTranscript(parsed)) {
+              console.warn(`[transcript] rejected translation blob${label ? ` (${label})` : ''}:`, cand.lang, `1 cue x ${String(parsed[0]?.text || '').length} chars`);
+              markTransient(':blob');
             }
             break;
           } catch (e) {
@@ -3593,20 +3599,18 @@ r.get('/transcript/:provider/:videoId', transcriptLimiter, async (req, res) => {
     const innertubeCands = keepAllowed(normalized === 'youtube'
       ? firstPerLanguage(orderedTracks(meta, lang), 2)
       : orderedTracks(meta, lang).slice(0, 2));
-    // Server-side auto-translation only towards an allowed target language.
-    const tlangCands = normalized === 'youtube'
-      ? keepAllowed(translatedFallbacks(innertubeCands, lang)).slice(0, 1)
-      : [];
-    await tryCandidates(innertubeCands.concat(tlangCands), { retry429: false, label: 'innertube' });
+    await tryCandidates(innertubeCands, { retry429: false, label: 'innertube' });
+    let signedBase = [];
     if (!lines.length && normalized === 'youtube' && transcriptSource !== 'innertube-only') {
       // Étape 2 : URLs signées via dump yt-dlp (même quand InnerTube a déjà
       // découvert les pistes : ce sont les seules URLs fetchables en direct).
+      // Directes uniquement : le `&tlang=` attend l'étape 3 (un blob de
+      // traduction dégradée ne doit jamais éclipser une piste directe).
       try {
         const raw = await youtubedl(transcriptUrl, { dumpSingleJson: true, skipDownload: true, noWarnings: true, noCheckCertificates: true, ...ytdlpNetOpts() });
         const dump = (typeof raw === 'string') ? JSON.parse(raw || '{}') : (raw || {});
-        const signedBase = keepAllowed(firstPerLanguage(orderedTracks(dump, lang), 3));
-        const signedTlang = keepAllowed(translatedFallbacks(signedBase, lang)).slice(0, 1);
-        const signed = mergeTranscriptCandidates({ signed: signedBase.concat(signedTlang), innertube: [], max: 4 });
+        signedBase = keepAllowed(firstPerLanguage(orderedTracks(dump, lang), 3));
+        const signed = mergeTranscriptCandidates({ signed: signedBase, innertube: [], max: 3 });
         if (signed.length) {
           console.log(`[transcript] retry via signed yt-dlp urls (${signed.length} candidats)`);
           await tryCandidates(signed, { retry429: true, label: 'signed' });
@@ -3615,14 +3619,24 @@ r.get('/transcript/:provider/:videoId', transcriptLimiter, async (req, res) => {
         console.warn('[transcript] signed-url dump failed:', String(e?.message || e).slice(0, 150));
       }
     }
+    if (!lines.length && normalized === 'youtube') {
+      // Étape 3 : traduction serveur `&tlang=` vers la langue demandée, à
+      // partir des pistes directes qui ont répondu (jamais en premier :
+      // voir garde anti-blob ci-dessus).
+      const tlangCands = keepAllowed(translatedFallbacks(innertubeCands.concat(signedBase), lang)).slice(0, 2);
+      if (tlangCands.length) await tryCandidates(tlangCands, { retry429: false, label: 'tlang' });
+    }
     if (!lines || lines.length === 0) {
-      // Étape 3 : téléchargement via yt-dlp (gère impersonation + retries).
-      // Uniquement les langues autorisées.
+      // Étape 4 : téléchargement via yt-dlp (gère impersonation + retries).
+      // Uniquement les langues autorisées ; blob refusé comme en direct.
       try {
         const viaDlp = await transcriptViaYtDlp(transcriptUrl, [lang, chosenLang].filter(l => allowedSet.has(transcriptPrimary(l))), ytdlpNetOpts());
-        if (viaDlp && viaDlp.lines && viaDlp.lines.length && allowedSet.has(transcriptPrimary(viaDlp.lang))) {
+        if (viaDlp && viaDlp.lines && viaDlp.lines.length && !isBlobTranscript(viaDlp.lines) && allowedSet.has(transcriptPrimary(viaDlp.lang))) {
           lines = viaDlp.lines;
           if (viaDlp.lang) workedLang = viaDlp.lang;
+        } else if (isBlobTranscript(viaDlp?.lines)) {
+          console.warn('[transcript] rejected yt-dlp blob:', viaDlp?.lang);
+          sawEmpty = true;
         }
       } catch {}
     }
@@ -3642,6 +3656,19 @@ r.get('/transcript/:provider/:videoId', transcriptLimiter, async (req, res) => {
       });
     }
     lines = dedupeTranscriptLines(lines);
+    if (isBlobTranscript(lines)) {
+      // Jamais de cache ni de 200 sur un blob : réponse transitoire.
+      console.warn('[transcript] rejected post-dedupe blob');
+      return res.status(502).json({
+        available: false,
+        languages: visibleLanguages,
+        lines: [],
+        error: 'transcript_temporarily_unavailable',
+        retryable: true,
+        rateLimited: true,
+        retryAfterSec: 60,
+      });
+    }
     const result = { lang: workedLang, available: true, languages: visibleLanguages, lines };
     transcriptCacheSet(cacheKey, result);
     return res.json(result);

server/tests/transcript.test.mjs

@@ -20,6 +20,7 @@ import {
   looksLikeHtmlError,
   ensureFmtParam,
   isEmptyTimedTextBody,
+  isBlobTranscript,
   mergeTranscriptCandidates,
   capLines,
   normalizeTranscriptProvider,
@@ -252,14 +253,22 @@ describe('timedtext fetch guards (anti-429 datacenter)', () => {
     assert.equal(isEmptyTimedTextBody(JSON.stringify({ events: [{ tStartMs: 0, dDurationMs: 1, segs: [{ utf8: 'Hi' }] }] })), false);
     assert.equal(isEmptyTimedTextBody('WEBVTT\n\n00:00:00.000 --> 00:00:01.000\nHello\n'), false);
   });
-  it('mergeTranscriptCandidates prefers signed urls, dedupes, bounds', () => {
-    const s = { track: { url: 'https://x/signed?sig=1' }, lang: 'fr' };
+  it('mergeTranscriptCandidates prefers signed urls, dedupes, bounds', () => {    const s = { track: { url: 'https://x/signed?sig=1' }, lang: 'fr' };
     const dup = { track: { url: 'https://x/signed?sig=1' }, lang: 'fr' };
     const it = { track: { url: 'https://x/innertube?fmt=json3' }, lang: 'fr' };
     const out = mergeTranscriptCandidates({ signed: [s, dup], innertube: [it], max: 5 });
     assert.deepEqual(out.map((c) => c.track.url), ['https://x/signed?sig=1', 'https://x/innertube?fmt=json3']);
     assert.equal(mergeTranscriptCandidates({ signed: [s], innertube: [it], max: 1 }).length, 1);
   });
+  it('isBlobTranscript rejects single giant cues, accepts real lines', () => {
+    assert.equal(isBlobTranscript([{ t: 0, dur: 1, text: 'x'.repeat(8581) }]), true);
+    assert.equal(isBlobTranscript([{ t: 0, dur: 1, text: 'Bonjour' }]), false);
+    assert.equal(isBlobTranscript([
+      { t: 0, dur: 1, text: 'a' },
+      { t: 1, dur: 1, text: 'b' },
+    ]), false);
+    assert.equal(isBlobTranscript([]), false);
+  });
 });
 
 describe('capLines + normalizeTranscriptProvider', () => {  it('truncates very long transcripts', () => {

server/transcript.mjs

@@ -352,6 +352,19 @@ export function isEmptyTimedTextBody(text) {
   return s.trim().length < 32;
 }
 
+/**
+ * True when parsed lines look like a degraded server-side translation blob
+ * rather than real cues: a single giant cue (several KB) instead of
+ * timestamped lines. YouTube serves these under throttle on `&tlang=`
+ * requests (source-language text, untranslated, in ONE event). Caching or
+ * labeling them as the target language would poison the UI + cache.
+ * A legitimate single-cue transcript (very short video) is always tiny.
+ * Pure and testable offline.
+ */
+export function isBlobTranscript(lines) {
+  if (!Array.isArray(lines) || lines.length !== 1) return false;
+  return String(lines[0]?.text || '').length > 2000;
+}
 /**
  * Merge fetch candidates, signed yt-dlp URLs first (they carry a fresh
  * `signature`/`expire` and are the only ones plain fetch can still read),