feat: integrate token exchange into MCP server application

Wire up RFC 8693 token exchange throughout the MCP server to support
stateless per-request token conversion for external IdP scenarios.

Changes:

Authentication Flow:
- Add exchange_token_for_audience() for pure RFC 8693 exchange
- Update context_helper to use stateless token exchange
- Remove fallback to standard OAuth on exchange failure
- Make storage initialization lazy (only for delegation, not MCP tools)

Application Configuration:
- Add ENABLE_TOKEN_EXCHANGE environment variable support
- Skip provisioning tools when token exchange enabled
- Pass mcp_client_id to token broker for proper validation
- Update docker-compose.yml with token exchange config

Token Exchange Service:
- Add TOKEN_EXCHANGE_GRANT constant
- Implement exchange_token_for_audience() method
- Support both "mcp-server" and client_id audiences
- Lazy storage initialization for delegation scenarios
- Enhanced error handling and logging

Progressive Token Verifier:
- Add mcp_client_id parameter for external IdP validation
- Accept both "mcp-server" and configured client_id
- Support external IdP token verification

Key Behavior Changes:
- When ENABLE_TOKEN_EXCHANGE=true: Each MCP tool call triggers
  stateless token exchange (client token → Nextcloud token)
- When ENABLE_TOKEN_EXCHANGE=false: Uses pass-through mode
  (validates Flow 1 token and passes to Nextcloud)
- No provisioning tools registered in exchange mode
- No refresh tokens needed for request-time operations

This completes the token exchange implementation. The MCP server now
supports both pass-through (default) and exchange (opt-in) modes for
federated authentication architectures.

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

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

docker-compose.yml

@@ -165,6 +165,9 @@ services:
       - TOKEN_ENCRYPTION_KEY=ESF1BvEQdGYsCluwMx9Cxvw3uh5pFowPH7Rg_nIliyo=
       - TOKEN_STORAGE_DB=/app/data/tokens.db
 
+      # Token exchange (RFC 8693) - convert aud:nextcloud-mcp-server → aud:nextcloud
+      - ENABLE_TOKEN_EXCHANGE=true
+
       # OAuth scopes (optional - uses defaults if not specified)
       - NEXTCLOUD_OIDC_SCOPES=openid profile email offline_access notes:read notes:write calendar:read calendar:write contacts:read contacts:write cookbook:read cookbook:write deck:read deck:write tables:read tables:write files:read files:write sharing:read sharing:write todo:read todo:write