feat: add custom health check commands per app

Add configurable health check commands per app via a new
'healthcheck_command' field. When set, the command is passed
to Docker as a CMD-SHELL health check on the container.
When empty, the image's default health check is used.

Changes:
- Add migration 007 for healthcheck_command column on apps table
- Add HealthcheckCommand field to App model with full CRUD support
- Add buildHealthcheck() to docker client for CMD-SHELL config
- Pass health check command through CreateContainerOptions
- Add health check command input to app create/edit UI forms
- Extract optionalNullString helper to reduce handler complexity
- Update README features list

closes #81

internal/models/models_test.go

@@ -704,6 +704,72 @@ func TestAppGetWebhookEvents(t *testing.T) {
 	assert.Len(t, events, 1)
 }
 
+// App HealthcheckCommand Tests.
+
+func TestAppHealthcheckCommand(t *testing.T) {
+	t.Parallel()
+
+	t.Run("saves and loads healthcheck command", func(t *testing.T) {
+		t.Parallel()
+
+		testDB, cleanup := setupTestDB(t)
+		defer cleanup()
+
+		app := createTestApp(t, testDB)
+		app.HealthcheckCommand = sql.NullString{
+			String: "curl -f http://localhost:8080/healthz || exit 1",
+			Valid:  true,
+		}
+
+		err := app.Save(context.Background())
+		require.NoError(t, err)
+
+		found, err := models.FindApp(context.Background(), testDB, app.ID)
+		require.NoError(t, err)
+		require.NotNil(t, found)
+		assert.True(t, found.HealthcheckCommand.Valid)
+		assert.Equal(t, "curl -f http://localhost:8080/healthz || exit 1", found.HealthcheckCommand.String)
+	})
+
+	t.Run("null when not set", func(t *testing.T) {
+		t.Parallel()
+
+		testDB, cleanup := setupTestDB(t)
+		defer cleanup()
+
+		app := createTestApp(t, testDB)
+
+		found, err := models.FindApp(context.Background(), testDB, app.ID)
+		require.NoError(t, err)
+		require.NotNil(t, found)
+		assert.False(t, found.HealthcheckCommand.Valid)
+	})
+
+	t.Run("can be cleared", func(t *testing.T) {
+		t.Parallel()
+
+		testDB, cleanup := setupTestDB(t)
+		defer cleanup()
+
+		app := createTestApp(t, testDB)
+		app.HealthcheckCommand = sql.NullString{String: "true", Valid: true}
+
+		err := app.Save(context.Background())
+		require.NoError(t, err)
+
+		// Clear it
+		app.HealthcheckCommand = sql.NullString{}
+
+		err = app.Save(context.Background())
+		require.NoError(t, err)
+
+		found, err := models.FindApp(context.Background(), testDB, app.ID)
+		require.NoError(t, err)
+		require.NotNil(t, found)
+		assert.False(t, found.HealthcheckCommand.Valid)
+	})
+}
+
 // Cascade Delete Tests.
 
 //nolint:funlen // Test function with many assertions - acceptable for integration tests