feat: Implement ADR-005 unified token verifier to eliminate token passthrough vulnerability

Replace two non-compliant token verifiers (NextcloudTokenVerifier and
ProgressiveConsentTokenVerifier) with a single UnifiedTokenVerifier that properly
validates token audiences per MCP Security Best Practices specification.

The previous implementation had a critical security vulnerability where tokens
intended for the MCP server were passed directly to Nextcloud APIs without
proper audience validation (token passthrough anti-pattern). This violates
OAuth 2.0 security principles and the MCP specification.

Changes:
- Add UnifiedTokenVerifier supporting two compliant modes:
  * Multi-audience mode (default): Validates tokens contain BOTH MCP and
    Nextcloud audiences, enabling direct use without exchange
  * Token exchange mode (opt-in): Validates MCP audience only, exchanges
    for Nextcloud tokens via RFC 8693 with caching to minimize latency

- Remove token passthrough vulnerability from context.py and context_helper.py
- Implement token exchange caching (5-minute TTL default) to reduce network calls
- Add required environment variables for audience validation:
  * NEXTCLOUD_MCP_SERVER_URL - MCP server URL (used as audience)
  * NEXTCLOUD_RESOURCE_URI - Nextcloud resource identifier
  * TOKEN_EXCHANGE_CACHE_TTL - Cache TTL for exchanged tokens

- Update docker-compose.yml with resource URI configuration for both OAuth modes
- Add comprehensive test suite (29 tests) covering both authentication modes
- Remove legacy NextcloudTokenVerifier and ProgressiveConsentTokenVerifier

Security improvements:
- Eliminates token passthrough anti-pattern
- Enforces proper audience separation between MCP and Nextcloud
- Complies with MCP Security Best Practices and RFC 8707/8693
- Maintains performance with token exchange caching

Test results: 65/65 unit tests passed, 5/5 smoke tests passed

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

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

docker-compose.yml

@@ -96,6 +96,7 @@ services:
       # OIDC_CLIENT_ID not set - uses Dynamic Client Registration (DCR)
       - NEXTCLOUD_HOST=http://app:80
       - NEXTCLOUD_MCP_SERVER_URL=http://localhost:8001
+      - NEXTCLOUD_RESOURCE_URI=http://localhost:8080  # ADR-005: Nextcloud resource identifier for audience validation
       - NEXTCLOUD_PUBLIC_ISSUER_URL=http://localhost:8080
       - NEXTCLOUD_OIDC_SCOPES=openid profile email 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
 
@@ -104,8 +105,9 @@ services:
       - TOKEN_ENCRYPTION_KEY=ESF1BvEQdGYsCluwMx9Cxvw3uh5pFowPH7Rg_nIliyo=
       - TOKEN_STORAGE_DB=/app/data/tokens.db
 
-      # ADR-004: Use Hybrid Flow (server intercepts OAuth callback)
-      # Set to false to enable Hybrid Flow tests - server stores refresh token and issues MCP codes
+      # ADR-005: Multi-audience mode (default - ENABLE_TOKEN_EXCHANGE=false)
+      # Tokens must contain BOTH MCP and Nextcloud audiences
+      # No token exchange needed - tokens work for both MCP auth and Nextcloud APIs
 
       # NO admin credentials - using OAuth with Dynamic Client Registration (DCR)
       # Client credentials registered via RFC 7591 and stored in volume
@@ -159,6 +161,7 @@ services:
       # Nextcloud API endpoint (for accessing APIs with validated token)
       - NEXTCLOUD_HOST=http://app:80
       - NEXTCLOUD_MCP_SERVER_URL=http://localhost:8002
+      - NEXTCLOUD_RESOURCE_URI=http://localhost:8080  # ADR-005: Nextcloud resource identifier for audience validation
       - NEXTCLOUD_PUBLIC_ISSUER_URL=http://localhost:8888/realms/nextcloud-mcp
 
       # Refresh token storage (ADR-002 Tier 1 & 2)
@@ -166,8 +169,11 @@ services:
       - TOKEN_ENCRYPTION_KEY=ESF1BvEQdGYsCluwMx9Cxvw3uh5pFowPH7Rg_nIliyo=
       - TOKEN_STORAGE_DB=/app/data/tokens.db
 
-      # Token exchange (RFC 8693) - convert aud:nextcloud-mcp-server → aud:nextcloud
+      # ADR-005: Token exchange mode (RFC 8693)
+      # Exchange MCP tokens (aud: nextcloud-mcp-server) for Nextcloud tokens (aud: http://localhost:8080)
+      # Provides strict audience separation between MCP session and Nextcloud API access
       - ENABLE_TOKEN_EXCHANGE=true
+      - TOKEN_EXCHANGE_CACHE_TTL=300  # Cache exchanged tokens for 5 minutes (default)
 
       # 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