feat: implement DNS namespace validation and remote URL uniqueness checks (#306)

## Summary

- Add reverse-DNS namespace format validation for server names
(namespace/name)
- Validate remote URLs match the DNS namespace derived from hostname  
- Prevent duplicate remote URLs across different servers
- Support localhost/development URLs without namespace restrictions
- Add comprehensive test coverage for validation logic
- Update database layer to support remote_url filtering

Fixes #180 by ensuring proper namespace governance and preventing URL
conflicts.

---------

Co-authored-by: Claude <noreply@anthropic.com>

internal/api/handlers/v0/publish.go

@@ -71,7 +71,7 @@ func RegisterPublishEndpoint(api huma.API, registry service.RegistryService, cfg
 		// Publish the server with extensions
 		publishedServer, err := registry.Publish(publishRequest)
 		if err != nil {
-			return nil, huma.Error500InternalServerError("Failed to publish server", err)
+			return nil, huma.Error400BadRequest("Failed to publish server", err)
 		}
 
 		// Return the published server in extension wrapper format

internal/api/handlers/v0/publish_integration_test.go

@@ -99,10 +99,10 @@ func TestPublishIntegration(t *testing.T) {
 		assert.Equal(t, publishReq.Server.VersionDetail.Version, response.Server.VersionDetail.Version)
 	})
 
-	t.Run("successful publish without auth (no prefix)", func(t *testing.T) {
+	t.Run("successful publish with none auth (no prefix)", func(t *testing.T) {
 		publishReq := model.PublishRequest{
 			Server: model.ServerDetail{
-				Name:        "test-mcp-server-no-auth",
+				Name:        "com.example/test-mcp-server-no-auth",
 				Description: "A test MCP server without authentication",
 				Repository: model.Repository{
 					URL:    "https://example.com/test-mcp-server",

internal/api/handlers/v0/publish_test.go

@@ -203,9 +203,9 @@ func TestPublishEndpoint(t *testing.T) {
 				},
 			},
 			setupMocks: func(registry *MockRegistryService) {
-				registry.On("Publish", mock.AnythingOfType("model.PublishRequest")).Return(nil, errors.New("database error"))
+				registry.On("Publish", mock.AnythingOfType("model.PublishRequest")).Return(nil, errors.New("cannot publish duplicate version"))
 			},
-			expectedStatus: http.StatusInternalServerError,
+			expectedStatus: http.StatusBadRequest,
 			expectedError:  "Failed to publish server",
 		},
 	}