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_container_test.go

@@ -2,6 +2,7 @@ package deploy_test
 
 import (
 	"context"
+	"database/sql"
 	"log/slog"
 	"os"
 	"testing"
@@ -43,3 +44,64 @@ func TestBuildContainerOptionsUsesImageID(t *testing.T) {
 		t.Errorf("expected Name=%q, got %q", "upaas-myapp", opts.Name)
 	}
 }
+
+func TestBuildContainerOptionsHealthcheckSet(t *testing.T) {
+	t.Parallel()
+
+	db := database.NewTestDatabase(t)
+
+	app := models.NewApp(db)
+	app.Name = "hc-app"
+	app.HealthcheckCommand = sql.NullString{
+		String: "curl -f http://localhost:8080/healthz || exit 1",
+		Valid:  true,
+	}
+
+	err := app.Save(context.Background())
+	if err != nil {
+		t.Fatalf("failed to save app: %v", err)
+	}
+
+	log := slog.New(slog.NewTextHandler(os.Stderr, nil))
+	svc := deploy.NewTestService(log)
+
+	opts, err := svc.BuildContainerOptionsExported(
+		context.Background(), app, "sha256:test",
+	)
+	if err != nil {
+		t.Fatalf("buildContainerOptions returned error: %v", err)
+	}
+
+	expected := "curl -f http://localhost:8080/healthz || exit 1"
+	if opts.HealthcheckCommand != expected {
+		t.Errorf("expected HealthcheckCommand=%q, got %q", expected, opts.HealthcheckCommand)
+	}
+}
+
+func TestBuildContainerOptionsHealthcheckEmpty(t *testing.T) {
+	t.Parallel()
+
+	db := database.NewTestDatabase(t)
+
+	app := models.NewApp(db)
+	app.Name = "no-hc-app"
+
+	err := app.Save(context.Background())
+	if err != nil {
+		t.Fatalf("failed to save app: %v", err)
+	}
+
+	log := slog.New(slog.NewTextHandler(os.Stderr, nil))
+	svc := deploy.NewTestService(log)
+
+	opts, err := svc.BuildContainerOptionsExported(
+		context.Background(), app, "sha256:test",
+	)
+	if err != nil {
+		t.Fatalf("buildContainerOptions returned error: %v", err)
+	}
+
+	if opts.HealthcheckCommand != "" {
+		t.Errorf("expected empty HealthcheckCommand, got %q", opts.HealthcheckCommand)
+	}
+}