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

db/migrations/20260927_add_oauth_connections.sql

@@ -0,0 +1,16 @@
+-- Connexions OAuth (import favoris / abonnements Google + Twitch).
+CREATE TABLE IF NOT EXISTS oauth_connections (
+  user_id TEXT NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+  provider TEXT NOT NULL,
+  external_user_id TEXT,
+  display_name TEXT,
+  avatar_url TEXT,
+  access_token TEXT,
+  refresh_token TEXT,
+  expires_at INTEGER,
+  scopes TEXT,
+  created_at INTEGER NOT NULL,
+  updated_at INTEGER NOT NULL,
+  PRIMARY KEY (user_id, provider)
+);
+CREATE INDEX IF NOT EXISTS idx_oauth_connections_user ON oauth_connections(user_id);