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)

package.json

@@ -9,6 +9,10 @@
     "preview": "ng serve --configuration=production",
     "api": "node --env-file=.env.local ./server/index.mjs",
     "api:watch": "node --watch --env-file=.env.local ./server/index.mjs",
+    "mcp": "node ./mcp/server.mjs",
+    "mcp:dev": "node --env-file=.env.local ./mcp/server.mjs",
+    "test:mcp": "node --test ./mcp/server.test.mjs ./mcp/tools.test.mjs",
+    "test:api": "node --test ./server/tests/api_coverage.test.mjs",
     "test:playlists": "node ./server/tests/playlist_visibility.test.mjs",
     "test:preferences": "node ./server/tests/preferences.test.mjs",
     "test:search-e2e": "node ./server/tests/search.e2e.test.mjs",