feat: Add Keycloak external IdP integration with custom scopes

Add comprehensive support for using Keycloak as an external identity
provider with Nextcloud custom scopes. This enables testing of ADR-002
external IdP integration patterns.

**Keycloak Realm Configuration:**
- Add frontendUrl attribute to issue tokens with public issuer URL
- Define 18 Nextcloud custom client scopes (notes:read/write,
  calendar:read/write, contacts:read/write, cookbook:read/write,
  deck:read/write, tables:read/write, files:read/write,
  sharing:read/write, todo:read/write)
- Add all custom scopes to nextcloud-mcp-server client optional scopes
- Scopes include consent screen text for user-friendly OAuth flow

**MCP Server Configuration:**
- Add OIDC_JWKS_URI environment variable support
- Implement JWKS URI override logic for Docker networking
- Update NEXTCLOUD_PUBLIC_ISSUER_URL to include full realm path
- Enable MCP server to fetch JWKS from internal Docker network

**Test Infrastructure:**
- Add keycloak_oauth_client_credentials fixture (session-scoped)
- Add keycloak_oauth_token fixture with Playwright automation
- Implement PKCE (S256) support for Keycloak OAuth flow
- Add nc_mcp_keycloak_client fixture for MCP testing
- Create comprehensive test suite in test_keycloak_external_idp.py

**Tests Created:**
- test_keycloak_oauth_token_acquisition: Token acquisition via Playwright
- test_keycloak_oauth_client_credentials_discovery: OIDC discovery
- test_mcp_client_connects_to_keycloak_server: MCP connectivity
- test_external_idp_server_initialization: Server auto-detection
- test_external_idp_token_validation: Token validation flow
- test_tools_work_with_keycloak_token: End-to-end tool execution
- test_keycloak_token_persistence: Multi-operation token reuse
- test_user_auto_provisioning: Nextcloud user provisioning
- test_scope_filtering_with_keycloak: Scope-based tool filtering
- test_keycloak_error_handling: Error handling
- test_external_idp_architecture: Architecture documentation

**Current Status:**
- ✅ Keycloak realm configuration complete
- ✅ Custom scopes defined and available
- ✅ OAuth token acquisition working (1 test passing)
- ⚠️  Token validation needs additional work (external IdP userinfo)

**Files Modified:**
- keycloak/realm-export.json: Realm configuration with scopes
- tests/conftest.py: Keycloak OAuth fixtures (+285 lines)
- tests/server/oauth/test_keycloak_external_idp.py: New test suite
- docker-compose.yml: OIDC_JWKS_URI and issuer configuration
- nextcloud_mcp_server/app.py: JWKS URI override logic

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

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

nextcloud_mcp_server/app.py

@@ -481,6 +481,13 @@ async def setup_oauth_config():
     introspection_uri = discovery.get("introspection_endpoint")
     registration_endpoint = discovery.get("registration_endpoint")
 
+    # Allow overriding JWKS URI (useful when running in Docker with frontendUrl)
+    # Example: frontendUrl=http://localhost:8888 but MCP server needs http://keycloak:8080
+    jwks_uri_override = os.getenv("OIDC_JWKS_URI")
+    if jwks_uri_override:
+        logger.info(f"OIDC_JWKS_URI override: {jwks_uri} → {jwks_uri_override}")
+        jwks_uri = jwks_uri_override
+
     logger.info("OIDC endpoints discovered:")
     logger.info(f"  Issuer: {issuer}")
     logger.info(f"  Userinfo: {userinfo_uri}")