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/migrations_goose/20261115000000_preview_environments_runtime.sql

@@ -0,0 +1,11 @@
+-- +goose Up
+ALTER TABLE public.preview_environments
+    ADD COLUMN IF NOT EXISTS preview_service_id uuid REFERENCES public.services(id) ON DELETE SET NULL;
+
+CREATE INDEX IF NOT EXISTS idx_preview_environments_expiry
+    ON public.preview_environments (expires_at)
+    WHERE status NOT IN ('expired', 'stopped');
+
+-- +goose Down
+DROP INDEX IF EXISTS public.idx_preview_environments_expiry;
+ALTER TABLE public.preview_environments DROP COLUMN IF EXISTS preview_service_id;