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/deploy/deploy.go

@@ -1094,14 +1094,20 @@ func (svc *Service) buildContainerOptions(
 		network = app.DockerNetwork.String
 	}
 
+	healthcheckCmd := ""
+	if app.HealthcheckCommand.Valid {
+		healthcheckCmd = app.HealthcheckCommand.String
+	}
+
 	return docker.CreateContainerOptions{
-		Name:    "upaas-" + app.Name,
-		Image:   imageID.String(),
-		Env:     envMap,
-		Labels:  buildLabelMap(app, labels),
-		Volumes: buildVolumeMounts(volumes),
-		Ports:   buildPortMappings(ports),
-		Network: network,
+		Name:               "upaas-" + app.Name,
+		Image:              imageID.String(),
+		Env:                envMap,
+		Labels:             buildLabelMap(app, labels),
+		Volumes:            buildVolumeMounts(volumes),
+		Ports:              buildPortMappings(ports),
+		Network:            network,
+		HealthcheckCommand: healthcheckCmd,
 	}, nil
 }