feat: Add Keycloak OAuth provider support with refresh token storage

Implements Keycloak as an external OIDC provider following ADR-002
architecture for background job authentication using offline_access.

## Features

- Keycloak OAuth provider with PKCE and offline_access support
- Refresh token storage with Fernet encryption
- Token verifier for both JWT and opaque tokens
- Multi-client validation (realm-level trust)
- Sample configuration for Keycloak integration

## Implementation

### OAuth Provider (keycloak_oauth.py)
- Authorization Code Flow with PKCE
- Refresh token exchange
- OIDC discovery endpoint support
- Token validation with JWKS

### Token Storage (refresh_token_storage.py)
- Encrypted storage using Fernet symmetric encryption
- SQLite backend for persistence
- Token rotation support
- Per-user token management

### Token Verifier Updates
- Support both JWT (self-encoded) and opaque tokens
- JWKS-based JWT signature verification
- Introspection endpoint fallback for opaque tokens
- Scope extraction from both token types

### Configuration
- .env.keycloak.sample: Example configuration with Keycloak URLs
- docs/keycloak-multi-client-validation.md: Realm-level validation documentation
- app-hooks/post-installation/10-install-user_oidc-app.sh: Updated dependencies

## Architecture Notes

- MCP Server is a protected resource (requires OAuth)
- MCP Client initiates OAuth flow and shares refresh tokens
- Refresh tokens enable background operations without admin credentials
- Supports future token exchange delegation when Keycloak implements it

## References

- ADR-002: Vector Database Background Sync Authentication
- RFC 6749: OAuth 2.0 (offline_access, refresh tokens)
- RFC 7517: JSON Web Key (JWK)

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

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

nextcloud_mcp_server/auth/token_verifier.py

@@ -168,17 +168,23 @@ class NextcloudTokenVerifier(TokenVerifier):
             signing_key = self._jwks_client.get_signing_key_from_jwt(token)
 
             # Verify and decode JWT
+            # Accept tokens with audience: "mcp-server" or ["mcp-server", "nextcloud"]
+            # This allows:
+            # 1. Tokens from MCP clients (aud: "mcp-server")
+            # 2. Tokens for Nextcloud APIs (aud: "nextcloud")
+            # 3. Tokens for both (aud: ["mcp-server", "nextcloud"])
             payload = jwt.decode(
                 token,
                 signing_key.key,
                 algorithms=["RS256"],
                 issuer=self.issuer,
+                audience=["mcp-server", "nextcloud"],  # Accept either audience
                 options={
                     "verify_signature": True,
                     "verify_exp": True,
                     "verify_iat": True,
                     "verify_iss": True if self.issuer else False,
-                    "verify_aud": False,  # Skip audience validation for Bearer tokens
+                    "verify_aud": True,  # Enable audience validation
                 },
             )