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/models_test.go

@@ -23,6 +23,7 @@ const (
 	testBranch    = "main"
 	testValue     = "value"
 	testEventType = "push"
+	testUser      = "user"
 )
 
 func setupTestDB(t *testing.T) (*database.Database, func()) {
@@ -704,6 +705,127 @@ func TestAppGetWebhookEvents(t *testing.T) {
 	assert.Len(t, events, 1)
 }
 
+// RegistryCredential Tests.
+
+func TestRegistryCredentialCreateAndFind(t *testing.T) {
+	t.Parallel()
+
+	testDB, cleanup := setupTestDB(t)
+	defer cleanup()
+
+	app := createTestApp(t, testDB)
+
+	cred := models.NewRegistryCredential(testDB)
+	cred.AppID = app.ID
+	cred.Registry = "registry.example.com"
+	cred.Username = "myuser"
+	cred.Password = "mypassword"
+
+	err := cred.Save(context.Background())
+	require.NoError(t, err)
+	assert.NotZero(t, cred.ID)
+
+	creds, err := models.FindRegistryCredentialsByAppID(
+		context.Background(), testDB, app.ID,
+	)
+	require.NoError(t, err)
+	require.Len(t, creds, 1)
+	assert.Equal(t, "registry.example.com", creds[0].Registry)
+	assert.Equal(t, "myuser", creds[0].Username)
+	assert.Equal(t, "mypassword", creds[0].Password)
+}
+
+func TestRegistryCredentialUpdate(t *testing.T) {
+	t.Parallel()
+
+	testDB, cleanup := setupTestDB(t)
+	defer cleanup()
+
+	app := createTestApp(t, testDB)
+
+	cred := models.NewRegistryCredential(testDB)
+	cred.AppID = app.ID
+	cred.Registry = "old.registry.com"
+	cred.Username = "olduser"
+	cred.Password = "oldpass"
+
+	err := cred.Save(context.Background())
+	require.NoError(t, err)
+
+	cred.Registry = "new.registry.com"
+	cred.Username = "newuser"
+	cred.Password = "newpass"
+
+	err = cred.Save(context.Background())
+	require.NoError(t, err)
+
+	found, err := models.FindRegistryCredential(context.Background(), testDB, cred.ID)
+	require.NoError(t, err)
+	require.NotNil(t, found)
+	assert.Equal(t, "new.registry.com", found.Registry)
+	assert.Equal(t, "newuser", found.Username)
+	assert.Equal(t, "newpass", found.Password)
+}
+
+func TestRegistryCredentialDelete(t *testing.T) {
+	t.Parallel()
+
+	testDB, cleanup := setupTestDB(t)
+	defer cleanup()
+
+	app := createTestApp(t, testDB)
+
+	cred := models.NewRegistryCredential(testDB)
+	cred.AppID = app.ID
+	cred.Registry = "delete.registry.com"
+	cred.Username = testUser
+	cred.Password = "pass"
+
+	err := cred.Save(context.Background())
+	require.NoError(t, err)
+
+	err = cred.Delete(context.Background())
+	require.NoError(t, err)
+
+	creds, err := models.FindRegistryCredentialsByAppID(
+		context.Background(), testDB, app.ID,
+	)
+	require.NoError(t, err)
+	assert.Empty(t, creds)
+}
+
+func TestRegistryCredentialFindByIDNotFound(t *testing.T) {
+	t.Parallel()
+
+	testDB, cleanup := setupTestDB(t)
+	defer cleanup()
+
+	found, err := models.FindRegistryCredential(context.Background(), testDB, 99999)
+	require.NoError(t, err)
+	assert.Nil(t, found)
+}
+
+func TestAppGetRegistryCredentials(t *testing.T) {
+	t.Parallel()
+
+	testDB, cleanup := setupTestDB(t)
+	defer cleanup()
+
+	app := createTestApp(t, testDB)
+
+	cred := models.NewRegistryCredential(testDB)
+	cred.AppID = app.ID
+	cred.Registry = "ghcr.io"
+	cred.Username = testUser
+	cred.Password = "token"
+	_ = cred.Save(context.Background())
+
+	creds, err := app.GetRegistryCredentials(context.Background())
+	require.NoError(t, err)
+	assert.Len(t, creds, 1)
+	assert.Equal(t, "ghcr.io", creds[0].Registry)
+}
+
 // Cascade Delete Tests.
 
 //nolint:funlen // Test function with many assertions - acceptable for integration tests
@@ -749,6 +871,13 @@ func TestCascadeDelete(t *testing.T) {
 		deploy.Status = models.DeploymentStatusSuccess
 		_ = deploy.Save(context.Background())
 
+		regCred := models.NewRegistryCredential(testDB)
+		regCred.AppID = app.ID
+		regCred.Registry = "registry.example.com"
+		regCred.Username = testUser
+		regCred.Password = "pass"
+		_ = regCred.Save(context.Background())
+
 		// Delete app.
 		err := app.Delete(context.Background())
 		require.NoError(t, err)
@@ -778,6 +907,11 @@ func TestCascadeDelete(t *testing.T) {
 			context.Background(), testDB, app.ID, 10,
 		)
 		assert.Empty(t, deployments)
+
+		regCreds, _ := models.FindRegistryCredentialsByAppID(
+			context.Background(), testDB, app.ID,
+		)
+		assert.Empty(t, regCreds)
 	})
 }