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/models/app.go

@@ -119,6 +119,11 @@ func (a *App) GetWebhookEvents(
 	return FindWebhookEventsByAppID(ctx, a.db, a.ID, limit)
 }
 
+// GetRegistryCredentials returns all registry credentials for the app.
+func (a *App) GetRegistryCredentials(ctx context.Context) ([]*RegistryCredential, error) {
+	return FindRegistryCredentialsByAppID(ctx, a.db, a.ID)
+}
+
 func (a *App) exists(ctx context.Context) bool {
 	if a.ID == "" {
 		return false