feat: add private Docker registry authentication for base images

Add per-app registry credentials that are passed to Docker during image
builds, allowing apps to use base images from private registries.

- New registry_credentials table (migration 007)
- RegistryCredential model with full CRUD operations
- Docker client passes AuthConfigs to ImageBuild when credentials exist
- Deploy service fetches app registry credentials before builds
- Web UI section for managing registry credentials (add/edit/delete)
- Comprehensive unit tests for model and auth config builder
- README updated to list the feature

internal/server/routes.go

@@ -98,6 +98,11 @@ func (s *Server) SetupRoutes() {
 			// Ports
 			r.Post("/apps/{id}/ports", s.handlers.HandlePortAdd())
 			r.Post("/apps/{id}/ports/{portID}/delete", s.handlers.HandlePortDelete())
+
+			// Registry Credentials
+			r.Post("/apps/{id}/registry-credentials", s.handlers.HandleRegistryCredentialAdd())
+			r.Post("/apps/{id}/registry-credentials/{credID}/edit", s.handlers.HandleRegistryCredentialEdit())
+			r.Post("/apps/{id}/registry-credentials/{credID}/delete", s.handlers.HandleRegistryCredentialDelete())
 		})
 	})