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/docker/validation_test.go

@@ -4,6 +4,7 @@ import (
 	"errors"
 	"log/slog"
 	"testing"
+	"time"
 )
 
 func TestValidBranchRegex(t *testing.T) {
@@ -146,3 +147,52 @@ func TestCloneRepoRejectsInjection(t *testing.T) { //nolint:funlen // table-driv
 		})
 	}
 }
+
+func TestBuildHealthcheck(t *testing.T) {
+	t.Parallel()
+
+	t.Run("creates CMD-SHELL health check", func(t *testing.T) {
+		t.Parallel()
+
+		cmd := "curl -f http://localhost:8080/healthz || exit 1"
+		hc := buildHealthcheck(cmd)
+
+		if len(hc.Test) != 2 {
+			t.Fatalf("expected 2 test elements, got %d", len(hc.Test))
+		}
+
+		if hc.Test[0] != "CMD-SHELL" {
+			t.Errorf("expected Test[0]=%q, got %q", "CMD-SHELL", hc.Test[0])
+		}
+
+		if hc.Test[1] != cmd {
+			t.Errorf("expected Test[1]=%q, got %q", cmd, hc.Test[1])
+		}
+	})
+
+	t.Run("sets expected intervals", func(t *testing.T) {
+		t.Parallel()
+
+		hc := buildHealthcheck("true")
+
+		expectedInterval := 30 * time.Second
+		if hc.Interval != expectedInterval {
+			t.Errorf("expected Interval=%v, got %v", expectedInterval, hc.Interval)
+		}
+
+		expectedTimeout := 10 * time.Second
+		if hc.Timeout != expectedTimeout {
+			t.Errorf("expected Timeout=%v, got %v", expectedTimeout, hc.Timeout)
+		}
+
+		expectedStartPeriod := 15 * time.Second
+		if hc.StartPeriod != expectedStartPeriod {
+			t.Errorf("expected StartPeriod=%v, got %v", expectedStartPeriod, hc.StartPeriod)
+		}
+
+		expectedRetries := 3
+		if hc.Retries != expectedRetries {
+			t.Errorf("expected Retries=%d, got %d", expectedRetries, hc.Retries)
+		}
+	})
+}