feat: remove global Nextcloud config, enforce member-specific storage providers

- Remove global Nextcloud settings from config
- Make NextcloudClient require explicit credentials
- Update for_member() to return None when no credentials
- Modify services to accept optional storage client
- Update routers to pass member storage to services
- Add 403 responses when no storage provider configured
- Update internal endpoints to use member storage credentials

This change enforces that each member must configure their own
Nextcloud storage provider. If no provider is configured,
file operations will return 403 FORBIDDEN instead of falling
back to global placeholders.

api/src/rehearsalhub/services/band.py

@@ -16,7 +16,7 @@ log = logging.getLogger(__name__)
 class BandService:
     def __init__(self, session: AsyncSession, storage: NextcloudClient | None = None) -> None:
         self._repo = BandRepository(session)
-        self._storage = storage or NextcloudClient()
+        self._storage = storage
 
     async def create_band(
         self,