Add auto-pause TTL and ping endpoint for sandbox inactivity management

Replace the existing auto-destroy TTL behavior with auto-pause: when a
sandbox exceeds its timeout_sec of inactivity, the TTL reaper now pauses
it (snapshot + teardown) instead of destroying it, preserving the ability
to resume later.

Key changes:
- TTL reaper calls Pause instead of Destroy, with fallback to Destroy if
  pause fails (e.g. Firecracker process already gone)
- New PingSandbox RPC resets the in-memory LastActiveAt timer
- New POST /v1/sandboxes/{id}/ping REST endpoint resets both agent memory
  and DB last_active_at
- ListSandboxes RPC now includes auto_paused_sandbox_ids so the reconciler
  can distinguish auto-paused sandboxes from crashed ones in a single call
- Reconciler polls every 5s (was 30s) and marks auto-paused as "paused"
  vs orphaned as "stopped"
- Resume RPC accepts timeout_sec from DB so TTL survives pause/resume cycles
- Reaper checks every 2s (was 10s) and uses a detached context to avoid
  incomplete pauses on app shutdown
- Default timeout_sec changed from 300 to 0 (no auto-pause unless requested)

cmd/control-plane/main.go

@@ -59,7 +59,7 @@ func main() {
 	srv := api.New(queries, agentClient, pool, []byte(cfg.JWTSecret))
 
 	// Start reconciler.
-	reconciler := api.NewReconciler(queries, agentClient, "default", 30*time.Second)
+	reconciler := api.NewReconciler(queries, agentClient, "default", 5*time.Second)
 	reconciler.Start(ctx)
 
 	httpServer := &http.Server{