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

@@ -1094,6 +1094,29 @@ func (svc *Service) buildContainerOptions(
 		network = app.DockerNetwork.String
 	}
 
+	cpuLimit, memoryLimit := extractResourceLimits(app)
+
+	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,
+		CPULimit:           cpuLimit,
+		MemoryLimit:        memoryLimit,
+		HealthcheckCommand: healthcheckCmd,
+	}, nil
+}
+
+// extractResourceLimits returns CPU and memory limits from the app model.
+func extractResourceLimits(app *models.App) (float64, int64) {
 	var cpuLimit float64
 
 	if app.CPULimit.Valid {
@@ -1106,17 +1129,7 @@ func (svc *Service) buildContainerOptions(
 		memoryLimit = app.MemoryLimit.Int64
 	}
 
-	return docker.CreateContainerOptions{
-		Name:        "upaas-" + app.Name,
-		Image:       imageID.String(),
-		Env:         envMap,
-		Labels:      buildLabelMap(app, labels),
-		Volumes:     buildVolumeMounts(volumes),
-		Ports:       buildPortMappings(ports),
-		Network:     network,
-		CPULimit:    cpuLimit,
-		MemoryLimit: memoryLimit,
-	}, nil
+	return cpuLimit, memoryLimit
 }
 
 func buildLabelMap(app *models.App, labels []*models.Label) map[string]string {