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>

docker-compose.yml

@@ -43,6 +43,11 @@ services:
       - MYSQL_USER=nextcloud
       - MYSQL_HOST=db
       - REDIS_HOST=redis
+    healthcheck:
+      test: ["CMD-SHELL", "curl -Ss http://localhost/status.php | grep '\"installed\":true' || exit 1"]
+      interval: 10s
+      timeout: 30s
+      retries: 30
 
   recipes:
     image: docker.io/library/nginx:alpine@sha256:b3c656d55d7ad751196f21b7fd2e8d4da9cb430e32f646adcf92441b72f82b14
@@ -137,11 +142,12 @@ services:
       - OIDC_DISCOVERY_URL=http://keycloak:8080/realms/nextcloud-mcp/.well-known/openid-configuration
       - OIDC_CLIENT_ID=nextcloud-mcp-server
       - OIDC_CLIENT_SECRET=mcp-secret-change-in-production
+      - OIDC_JWKS_URI=http://keycloak:8080/realms/nextcloud-mcp/protocol/openid-connect/certs
 
       # Nextcloud API endpoint (for accessing APIs with validated token)
       - NEXTCLOUD_HOST=http://app:80
       - NEXTCLOUD_MCP_SERVER_URL=http://localhost:8002
-      - NEXTCLOUD_PUBLIC_ISSUER_URL=http://localhost:8888
+      - NEXTCLOUD_PUBLIC_ISSUER_URL=http://localhost:8888/realms/nextcloud-mcp
 
       # Refresh token storage (ADR-002 Tier 1 & 2)
       - ENABLE_OFFLINE_ACCESS=true