Add template build system with admin panel, async workers, and FlattenRootfs RPC

Introduces an end-to-end template building pipeline: admins submit a recipe
(list of shell commands) via the dashboard, a Redis-backed worker pool spins
up a sandbox, executes each command, and produces either a full snapshot
(with healthcheck) or an image-only template (rootfs flattened via a new
FlattenRootfs host-agent RPC). Build progress and per-step logs are persisted
to a new template_builds table and polled by the frontend.

Backend:
- New FlattenRootfs RPC (proto + host agent + sandbox manager)
- BuildService with Redis queue (BLPOP) and configurable worker pool (default 2)
- Admin-only REST endpoints: POST/GET /v1/admin/builds, GET /v1/admin/builds/{id}
- Migration for template_builds table with JSONB logs and recipe columns
- sqlc queries for build CRUD and progress updates

Frontend:
- /admin/templates page with Templates + Builds tabs
- Create Template dialog with recipe textarea, healthcheck, specs
- Build history with expandable per-step logs, status badges, progress bars
- Auto-polling every 3s for active builds
- AdminSidebar updated with Templates nav item

cmd/control-plane/main.go

@@ -90,6 +90,10 @@ func main() {
 	// API server.
 	srv := api.New(queries, hostPool, hostScheduler, pool, rdb, []byte(cfg.JWTSecret), oauthRegistry, cfg.OAuthRedirectURL)
 
+	// Start template build workers (2 concurrent).
+	stopBuildWorkers := srv.BuildSvc.StartWorkers(ctx, 2)
+	defer stopBuildWorkers()
+
 	// Start host monitor (passive + active reconciliation every 30s).
 	monitor := api.NewHostMonitor(queries, hostPool, audit.New(queries), 30*time.Second)
 	monitor.Start(ctx)