fix: add 30s timeout to PB HTTP client; halve heartbeat tick interval

- storage/pocketbase.go: replace http.DefaultClient (no timeout) with a
  dedicated pbHTTPClient{Timeout: 30s} so a slow/hung PocketBase cannot
  stall the backend or runner indefinitely
- runner/asynq_runner.go: heartbeat ticker was firing at StaleTaskThreshold
  (2 min) == the Docker healthcheck deadline, so a single missed tick would
  mark the container unhealthy; halved to StaleTaskThreshold/2 (1 min)

backend/internal/runner/asynq_runner.go

@@ -78,7 +78,7 @@ func (r *Runner) runAsynq(ctx context.Context) error {
 	// Write /tmp/runner.alive every 30s so Docker healthcheck passes in asynq mode.
 	// This mirrors the heartbeat file behavior from the poll() loop.
 	go func() {
-		heartbeatTick := time.NewTicker(r.cfg.StaleTaskThreshold)
+		heartbeatTick := time.NewTicker(r.cfg.StaleTaskThreshold / 2)
 		defer heartbeatTick.Stop()
 		for {
 			select {