feat(sync): phase 1 - import Takeout + Watch Later YouTube bidirectionnel

- Takeout: POST /user/history/takeout (lots <=1000, validation videoId,
  upsert sans doublons), parsing local watch-history.json cote client
  (vie privee, dedup derniere date), progression par lots de 500
- Watch Later: GET/POST /oauth/google/watchlater (lecture readonly,
  push force-ssl), import vers playlist NewTube 'Watch Later (YouTube)',
  export playlist NewTube -> WL YouTube, writeGranted + invite reconnect
- Scopes Google elargis (youtube.force-ssl), aide /library/import a jour
  (limite historique API documentee)

server/oauth.mjs

@@ -21,8 +21,17 @@ const GOOGLE_SCOPES = [
   'openid',
   'https://www.googleapis.com/auth/userinfo.profile',
   'https://www.googleapis.com/auth/youtube.readonly',
+  // Écriture Watch Later (playlistItems.insert/delete). Lecture seule
+  // (abos, likes, WL) fonctionne sans lui ; le push exige un re-consentement.
+  'https://www.googleapis.com/auth/youtube.force-ssl',
 ].join(' ');
 
+/** Le jeton stocké permet-il l'écriture (push Watch Later) ? */
+export function hasGoogleWriteScope(scopes) {
+  const tokens = String(scopes || '').split(/\s+/).filter(Boolean);
+  return tokens.some((t) => t === 'https://www.googleapis.com/auth/youtube.force-ssl' || t === 'https://www.googleapis.com/auth/youtube');
+}
+
 const TWITCH_SCOPES = ['user:read:follows', 'user:read:subscriptions'].join(' ');
 
 // state -> { userId, provider, createdAt } (mémoire, 10 min).
@@ -298,6 +307,83 @@ export async function fetchGoogleLiked(accessToken, max = 25) {
   return out;
 }
 
+// -------------------- Google : Watch Later --------------------
+
+async function googleApiGet(accessToken, path, params = {}) {
+  const qs = new URLSearchParams();
+  for (const [k, v] of Object.entries(params)) {
+    if (v !== undefined && v !== null && v !== '') qs.set(k, String(v));
+  }
+  return getJson(`https://www.googleapis.com/youtube/v3/${path}?${qs.toString()}`, accessToken);
+}
+
+async function googleApiPost(accessToken, path, body) {
+  const resp = await fetch(`https://www.googleapis.com/youtube/v3/${path}`, {
+    method: 'POST',
+    headers: { Authorization: `Bearer ${accessToken}`, 'Content-Type': 'application/json' },
+    body: JSON.stringify(body || {}),
+  });
+  const data = await resp.json().catch(() => ({}));
+  if (!resp.ok) {
+    const err = new Error(`oauth_api_failed_${resp.status}`);
+    err.status = resp.status === 401 ? 401 : resp.status === 403 ? 403 : 502;
+    err.details = data;
+    throw err;
+  }
+  return data;
+}
+
+/** ID de la playlist "Regarder plus tard" du compte (lecture seule OK). */
+export async function fetchGoogleWatchLaterId(accessToken) {
+  const data = await googleApiGet(accessToken, 'channels', { part: 'contentDetails', mine: 'true' });
+  const item = Array.isArray(data?.items) ? data.items[0] : null;
+  const id = item?.contentDetails?.relatedPlaylists?.watchLater || '';
+  if (!id) throw Object.assign(new Error('watchlater_not_found'), { status: 502 });
+  return String(id);
+}
+
+/** Contenu de "Regarder plus tard" (lecture seule OK). */
+export async function fetchGoogleWatchLater(accessToken, max = 50) {
+  const playlistId = await fetchGoogleWatchLaterId(accessToken);
+  const out = [];
+  let pageToken = '';
+  while (out.length < max) {
+    const data = await googleApiGet(accessToken, 'playlistItems', {
+      part: 'snippet,contentDetails',
+      playlistId,
+      maxResults: Math.min(50, max - out.length),
+      ...(pageToken ? { pageToken } : {}),
+    });
+    for (const item of Array.isArray(data?.items) ? data.items : []) {
+      const vid = item?.contentDetails?.videoId || item?.snippet?.resourceId?.videoId || '';
+      if (!vid) continue;
+      const sn = item?.snippet || {};
+      out.push({
+        provider: 'youtube',
+        videoId: String(vid),
+        title: sn?.title || '',
+        thumbnail: sn?.thumbnails?.medium?.url || sn?.thumbnails?.default?.url || '',
+        playlistItemId: item?.id || null,
+      });
+    }
+    pageToken = data?.nextPageToken || '';
+    if (!pageToken) break;
+  }
+  return { playlistId, items: out };
+}
+
+/** Ajoute une vidéo à "Regarder plus tard" (exige le scope force-ssl). */
+export async function pushGoogleWatchLater(accessToken, videoId) {
+  const playlistId = await fetchGoogleWatchLaterId(accessToken);
+  const data = await googleApiPost(accessToken, 'playlistItems?part=snippet', {
+    snippet: {
+      playlistId,
+      resourceId: { kind: 'youtube#video', videoId: String(videoId) },
+    },
+  });
+  return { playlistItemId: data?.id || null };
+}
+
 // -------------------- Twitch --------------------
 
 function twitchClientId() {