feat(oauth): import favoris/abos Google (YouTube) et Twitch vers abonnements/likes

- Backend: server/oauth.mjs (auth URL, echange code, refresh, fetch
  subscriptions/likes Google, follows Twitch), table oauth_connections
  + migration, routes /api/oauth/:provider/{url,callback,preview,import},
  /api/oauth/{status,connections}
- Front: /library/import (connect, preview, import), service
  oauth-import, lien depuis Abonnements
- Config: GOOGLE_CLIENT_ID/SECRET/REDIRECT_URI, TWITCH_REDIRECT_URI,
  OAUTH_APP_BASE_URL documentes (.env.example, docker-compose),
  passthrough docker-compose.local.yml, roadmap README cochee

src/components/library/subscriptions/subscriptions.component.html

@@ -19,6 +19,12 @@
       >
         Rafraîchir
       </button>
+      <a
+        [routerLink]="['/library/import']"
+        class="rounded-md border border-slate-700 bg-slate-800 px-3 py-1.5 text-sm text-slate-200 transition hover:border-slate-500 hover:text-white"
+      >
+        ⬇ Importer (Google / Twitch)
+      </a>
     </div>
   </div>