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/service/app/app.go

@@ -46,13 +46,14 @@ func New(_ fx.Lifecycle, params ServiceParams) (*Service, error) {
 
 // CreateAppInput contains the input for creating an app.
 type CreateAppInput struct {
-	Name           string
-	RepoURL        string
-	Branch         string
-	DockerfilePath string
-	DockerNetwork  string
-	NtfyTopic      string
-	SlackWebhook   string
+	Name               string
+	RepoURL            string
+	Branch             string
+	DockerfilePath     string
+	DockerNetwork      string
+	NtfyTopic          string
+	SlackWebhook       string
+	HealthcheckCommand string
 }
 
 // CreateApp creates a new application with generated SSH keys and webhook secret.
@@ -100,6 +101,10 @@ func (svc *Service) CreateApp(
 		app.SlackWebhook = sql.NullString{String: input.SlackWebhook, Valid: true}
 	}
 
+	if input.HealthcheckCommand != "" {
+		app.HealthcheckCommand = sql.NullString{String: input.HealthcheckCommand, Valid: true}
+	}
+
 	saveErr := app.Save(ctx)
 	if saveErr != nil {
 		return nil, fmt.Errorf("failed to save app: %w", saveErr)
@@ -112,13 +117,14 @@ func (svc *Service) CreateApp(
 
 // UpdateAppInput contains the input for updating an app.
 type UpdateAppInput struct {
-	Name           string
-	RepoURL        string
-	Branch         string
-	DockerfilePath string
-	DockerNetwork  string
-	NtfyTopic      string
-	SlackWebhook   string
+	Name               string
+	RepoURL            string
+	Branch             string
+	DockerfilePath     string
+	DockerNetwork      string
+	NtfyTopic          string
+	SlackWebhook       string
+	HealthcheckCommand string
 }
 
 // UpdateApp updates an existing application.
@@ -144,6 +150,10 @@ func (svc *Service) UpdateApp(
 		String: input.SlackWebhook,
 		Valid:  input.SlackWebhook != "",
 	}
+	app.HealthcheckCommand = sql.NullString{
+		String: input.HealthcheckCommand,
+		Valid:  input.HealthcheckCommand != "",
+	}
 
 	saveErr := app.Save(ctx)
 	if saveErr != nil {