feat: add OpenAPI tags for improved SDK generation and API organization (#655)

## Summary
Adds OpenAPI tags to improve SDK generation and API documentation
organization, as discussed in #644.

## Changes
- **Go Code**: Define tag metadata (name + description) in
`internal/api/router/router.go` using Huma's `api.OpenAPI().Tags`
- **Reference Spec**: Update `docs/reference/api/openapi.yaml` to match
the generated spec
- **Tags Added**: servers, publish, auth, admin, health, ping

## Benefits
- ✅ Improved SDK generation with proper service grouping (e.g.,
`serversService`, `publishService`)
- ✅ Better API documentation organization in Scalar UI, Swagger UI, and
other tools
- ✅ Code-first approach ensures reference spec stays in sync via
compliance tests

## Testing
- ✅ Compliance test passes: `go test -v ./internal/api -run
TestOpenAPIEndpointCompliance`
- ✅ All 4 reference endpoints verified as implemented
- ✅ TypeScript SDK regenerated successfully with proper tag-based
grouping

## Related
Closes #644

internal/api/router/router.go

@@ -139,6 +139,18 @@ func NewHumaAPI(cfg *config.Config, registry service.RegistryService, mux *http.
 	// Create a new API using humago adapter for standard library
 	api := humago.New(mux, humaConfig)
 
+	// Add OpenAPI tag metadata with descriptions
+	api.OpenAPI().Tags = []*huma.Tag{
+		{
+			Name:        "servers",
+			Description: "Operations for discovering and retrieving MCP servers",
+		},
+		{
+			Name:        "publish",
+			Description: "Operations for publishing MCP servers to the registry",
+		},
+	}
+
 	// Add metrics middleware with options
 	api.UseMiddleware(MetricTelemetryMiddleware(metrics,
 		WithSkipPaths("/health", "/metrics", "/ping", "/docs"),