feat(mcp): serveur MCP stdio 7 outils + guide API/MCP + couverture tests

- mcp/server.mjs : search, suggest, details, trending, transcript,
  transcript_text, health_check (client fin vers API REST)
- docs/API_MCP_GUIDE.md : reference complete REST + MCP + transcript
- tests : server/tests/api_coverage.test.mjs (17 cas HTTP),
  mcp/tools.test.mjs (7 outils sur stub), scripts test:mcp/test:api
- fix(db): DELETE playlist 500 FK -> metrique avant delete + cleanup
  enfants + recordPlaylistMetric tolerant (observabilite)

server/db.mjs

@@ -771,14 +771,20 @@ export function listLikedVideos({ userId, limit = 100, q }) {
 }
 
 // -------------------- Playlists --------------------
+// Metrics are observability-only: they must never break the core operation
+// (e.g. a playlist DELETE must stay 204 even if the metrics insert fails).
 export function recordPlaylistMetric({ userId, playlistId, action, meta }) {
-  const id = cryptoRandomId();
-  const created_at = nowIso();
-  const meta_json = meta ? JSON.stringify(meta) : null;
-  db.prepare(`INSERT INTO playlist_metrics (id, user_id, playlist_id, action, meta_json, created_at)
-              VALUES (?, ?, ?, ?, ?, ?)`)
-    .run(id, userId, playlistId, action, meta_json, created_at);
-  return { id, created_at };
+  try {
+    const id = cryptoRandomId();
+    const created_at = nowIso();
+    const meta_json = meta ? JSON.stringify(meta) : null;
+    db.prepare(`INSERT INTO playlist_metrics (id, user_id, playlist_id, action, meta_json, created_at)
+                VALUES (?, ?, ?, ?, ?, ?)`)
+      .run(id, userId, playlistId, action, meta_json, created_at);
+    return { id, created_at };
+  } catch {
+    return null;
+  }
 }
 
 export function createPlaylist({ userId, title, description, thumbnail, isPrivate = true }) {
@@ -864,8 +870,13 @@ export function deletePlaylist({ userId, id }) {
   const cur = db.prepare(`SELECT * FROM playlists WHERE id = ?`).get(id);
   if (!cur) return { removed: false };
   if (cur.user_id !== userId) return 'forbidden';
+  // Record BEFORE the row disappears: playlist_metrics.playlist_id references
+  // playlists(id), so inserting after the DELETE violates the FK constraint.
+  try { recordPlaylistMetric({ userId, playlistId: id, action: 'delete' }); } catch {}
+  // Explicit child cleanup (works even on old DBs whose FK lacks ON DELETE CASCADE).
+  try { db.prepare(`DELETE FROM playlist_items WHERE playlist_id = ?`).run(id); } catch {}
+  try { db.prepare(`DELETE FROM playlist_metrics WHERE playlist_id = ?`).run(id); } catch {}
   const info = db.prepare(`DELETE FROM playlists WHERE id = ?`).run(id);
-  recordPlaylistMetric({ userId, playlistId: id, action: 'delete' });
   return { removed: (info.changes || 0) > 0 };
 }