feat: add API key bearer token authentication

- ApiKeyVerifier(TokenVerifier) — validates Authorization: Bearer <key>
  against UNRAID_MCP_API_KEY; guards against empty-key bypass
- _build_auth() replaces module-level _build_google_auth() call:
  returns MultiAuth(server=google, verifiers=[api_key]) when both set,
  GoogleProvider alone, ApiKeyVerifier alone, or None
- settings.py: add UNRAID_MCP_API_KEY + is_api_key_auth_configured()
  + api_key_auth_enabled in get_config_summary()
- run_server(): improved auth status logging for all three states
- tests/test_api_key_auth.py: 9 tests covering verifier + _build_auth
- .env.example: add UNRAID_MCP_API_KEY section
- docs/GOOGLE_OAUTH.md: add API Key section
- README.md / CLAUDE.md: rename section, document both auth methods
- Fix pre-existing: test_health.py patched cache_middleware/error_middleware
  now match renamed _cache_middleware/_error_middleware in server.py

tests/test_health.py

@@ -141,8 +141,8 @@ class TestHealthActions:
                 "unraid_mcp.subscriptions.utils._analyze_subscription_status",
                 return_value=(0, []),
             ),
-            patch("unraid_mcp.server.cache_middleware", mock_cache),
-            patch("unraid_mcp.server.error_middleware", mock_error),
+            patch("unraid_mcp.server._cache_middleware", mock_cache),
+            patch("unraid_mcp.server._error_middleware", mock_error),
         ):
             result = await tool_fn(action="health", subaction="diagnose")
         assert "subscriptions" in result