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/routes.go

@@ -111,6 +111,7 @@ func SetupRoutes(router *gin.Engine, db *database.DB, redis *database.Redis, cfg
 
 	// Sleep sweeper — scales idle services to zero and prunes wake containers.
 	StartSleepSweeper(context.Background(), db, dockerClient, deploymentEngine, cfg.Port)
+	StartPreviewSweeper(context.Background(), db, deploymentEngine)
 
 	// Audit retention — prunes rows past AUDIT_RETENTION_DAYS (default 90).
 	StartAuditRetention(context.Background(), db)