feat(vector-sync): enable background sync in OAuth mode

Add multi-user background vector synchronization when running in OAuth
mode with ENABLE_OFFLINE_ACCESS=true. Key changes:

Architecture (oauth_sync.py):
- User Manager task polls RefreshTokenStorage for provisioned users
- Per-user scanner tasks fetch documents using OAuth tokens
- Shared processor pool indexes documents from all users

Token Broker improvements:
- Accept client_id/client_secret instead of encryption_key
- Remove redundant token audience pre-validation (Nextcloud validates)
- Add _rewrite_token_endpoint for Docker internal URL routing
- Remove double-decryption (storage handles encryption internally)

Browser OAuth flow fixes:
- Add 'resource' parameter to request Nextcloud-scoped tokens
- Store and retrieve next_url for proper redirect after consent
- Rewrite token endpoint URLs for internal Docker access

Configuration:
- Add vector_sync_user_poll_interval setting (default: 60s)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

nextcloud_mcp_server/config.py

@@ -205,6 +205,7 @@ class Settings:
     vector_sync_scan_interval: int = 300  # seconds (5 minutes)
     vector_sync_processor_workers: int = 3
     vector_sync_queue_max_size: int = 10000
+    vector_sync_user_poll_interval: int = 60  # seconds - OAuth mode user discovery
 
     # Qdrant settings (mutually exclusive modes)
     qdrant_url: Optional[str] = None  # Network mode: http://qdrant:6333
@@ -391,6 +392,9 @@ def get_settings() -> Settings:
         vector_sync_queue_max_size=int(
             os.getenv("VECTOR_SYNC_QUEUE_MAX_SIZE", "10000")
         ),
+        vector_sync_user_poll_interval=int(
+            os.getenv("VECTOR_SYNC_USER_POLL_INTERVAL", "60")
+        ),
         # Qdrant settings
         qdrant_url=os.getenv("QDRANT_URL"),
         qdrant_location=os.getenv("QDRANT_LOCATION"),