feat(preview): real preview environments — clone, deploy, TTL, promote (#67)

Preview environments were bookkeeping only: a fabricated deployment row
marked "running" and a fake *.preview.containr.local URL. Now a preview
is a real service clone — full config copy (source, env, volumes, access),
git_branch pinned to the requested branch, its own preview hostname under
the clone's node default domain, and a real deployment through the queue.

- cloneServiceRow extracted from handleCloneService for reuse
- preview_environments.preview_service_id links to the clone
- minute sweeper: expires past-TTL previews (removes clone runtime + row)
  and syncs preview status from the clone's live service status
- delete/cleanup paths tear the clone down, not just the row
- promote redeploys the SOURCE service at the preview branch and retires
  the clone — a real deployment, not a pending stub

Verified live: create → clone → deploy → running container; TTL expiry →
clone + container gone; promote → real deploy on the source, preview
stopped. Unit test covers the DNS-safe clone naming.

Generated with [Devin](https://devin.ai)

Co-authored-by: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

app/backend/internal/api/preview_environments_test.go

@@ -0,0 +1,30 @@
+package api
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestPreviewServiceName(t *testing.T) {
+	cases := []struct {
+		svc, branch string
+		want        string
+	}{
+		{"whoami-web", "feature-x", "whoami-web-feature-x-preview"},
+		{"My App", "feat/JARV-123_foo", "my-app-feat-jarv-123-foo-preview"},
+		{"svc", "", "svc-preview"},
+	}
+	for _, tc := range cases {
+		got := previewServiceName(tc.svc, tc.branch)
+		if got != tc.want {
+			t.Fatalf("previewServiceName(%q,%q) = %q, want %q", tc.svc, tc.branch, got, tc.want)
+		}
+		if len(got) > 63 {
+			t.Fatalf("name exceeds DNS label limit: %q", got)
+		}
+	}
+	// A name that sanitizes to nothing must still be usable.
+	if got := previewServiceName("!!!", "%%%"); !strings.HasPrefix(got, "preview-") {
+		t.Fatalf("expected fallback name, got %q", got)
+	}
+}