refactor: consolidate database storage for webhooks and OAuth tokens

Refactored the storage system to use a unified SQLite database for both
webhook tracking and OAuth token storage, available in both BasicAuth
and OAuth modes.

Changes:
- Renamed refresh_token_storage.py → storage.py
- Made TOKEN_ENCRYPTION_KEY optional (only required for OAuth token ops)
- Added registered_webhooks table with schema versioning
- Added webhook storage methods (store, get, delete, list, clear)
- Initialize storage in both BasicAuth and OAuth modes
- Updated webhook routes to persist registrations in database
- Database-first pattern for webhook status checks (performance)
- Updated all imports across codebase

Storage Behavior:
- Database created automatically at startup if needed
- Existing databases detected and reused
- Server fails fast if database initialization fails
- No migrations needed (OAuth feature is experimental)

Testing:
- Added 13 comprehensive unit tests for webhook storage
- All 118 unit tests pass
- All 5 smoke tests pass
- Verified fail-fast behavior on initialization errors

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

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

nextcloud_mcp_server/server/oauth_tools.py

@@ -18,7 +18,7 @@ from mcp.server.fastmcp import Context
 from pydantic import BaseModel, Field
 
 from nextcloud_mcp_server.auth import require_scopes
-from nextcloud_mcp_server.auth.refresh_token_storage import RefreshTokenStorage
+from nextcloud_mcp_server.auth.storage import RefreshTokenStorage
 from nextcloud_mcp_server.auth.token_broker import TokenBrokerService
 from nextcloud_mcp_server.auth.userinfo_routes import _query_idp_userinfo