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/handlers/app.go

@@ -57,15 +57,17 @@ func (h *Handlers) HandleAppCreate() http.HandlerFunc { //nolint:funlen // valid
 		dockerNetwork := request.FormValue("docker_network")
 		ntfyTopic := request.FormValue("ntfy_topic")
 		slackWebhook := request.FormValue("slack_webhook")
+		healthcheckCommand := request.FormValue("healthcheck_command")
 
 		data := h.addGlobals(map[string]any{
-			"Name":           name,
-			"RepoURL":        repoURL,
-			"Branch":         branch,
-			"DockerfilePath": dockerfilePath,
-			"DockerNetwork":  dockerNetwork,
-			"NtfyTopic":      ntfyTopic,
-			"SlackWebhook":   slackWebhook,
+			"Name":               name,
+			"RepoURL":            repoURL,
+			"Branch":             branch,
+			"DockerfilePath":     dockerfilePath,
+			"DockerNetwork":      dockerNetwork,
+			"NtfyTopic":          ntfyTopic,
+			"SlackWebhook":       slackWebhook,
+			"HealthcheckCommand": healthcheckCommand,
 		}, request)
 
 		if name == "" || repoURL == "" {
@@ -102,13 +104,14 @@ func (h *Handlers) HandleAppCreate() http.HandlerFunc { //nolint:funlen // valid
 		createdApp, createErr := h.appService.CreateApp(
 			request.Context(),
 			app.CreateAppInput{
-				Name:           name,
-				RepoURL:        repoURL,
-				Branch:         branch,
-				DockerfilePath: dockerfilePath,
-				DockerNetwork:  dockerNetwork,
-				NtfyTopic:      ntfyTopic,
-				SlackWebhook:   slackWebhook,
+				Name:               name,
+				RepoURL:            repoURL,
+				Branch:             branch,
+				DockerfilePath:     dockerfilePath,
+				DockerNetwork:      dockerNetwork,
+				NtfyTopic:          ntfyTopic,
+				SlackWebhook:       slackWebhook,
+				HealthcheckCommand: healthcheckCommand,
 			},
 		)
 		if createErr != nil {
@@ -260,6 +263,7 @@ func (h *Handlers) HandleAppUpdate() http.HandlerFunc { //nolint:funlen // valid
 		application.DockerNetwork = optionalNullString(request.FormValue("docker_network"))
 		application.NtfyTopic = optionalNullString(request.FormValue("ntfy_topic"))
 		application.SlackWebhook = optionalNullString(request.FormValue("slack_webhook"))
+		application.HealthcheckCommand = optionalNullString(request.FormValue("healthcheck_command"))
 
 		limitsErr := applyResourceLimits(application, request)
 		if limitsErr != "" {