feat(outbox): implement concurrent claim mechanism with UPDATE RETURNING + SKIP LOCKED

- Add migration 0004 to introduce 'claiming' status and timeout index
- Add StatusClaiming to platformevent domain and allow it in Validate()
- Rewrite ListDue as transactional UPDATE ... RETURNING with FOR UPDATE SKIP LOCKED
- Add ReleaseStaleClaims to reset expired claiming events back to retrying
- Worker Start() now runs a 30s ticker for stale claim recovery (5m timeout)
- Update stubEventStore in tests to satisfy new EventStore interface

Refs: D-02

internal/service/platformdelivery/worker.go

@@ -20,6 +20,7 @@ type EventStore interface {
 	RecordDeliveryAttempt(ctx context.Context, eventID string, attemptNo int, responseStatus int, responseBody string, errorMessage string) error
 	MarkRetry(ctx context.Context, eventID string, attemptCount int, nextAttemptAt time.Time, lastError string) error
 	MarkDeadLetter(ctx context.Context, eventID string, attemptCount int, finalError string) error
+	ReleaseStaleClaims(ctx context.Context, timeout time.Duration) (int, error)
 }
 
 type Worker struct {
@@ -31,6 +32,7 @@ type Worker struct {
 	MaxRetries    int
 	BatchSize     int
 	PollInterval  time.Duration
+	ClaimTimeout  time.Duration
 	RetrySchedule []time.Duration
 	Now           func() time.Time
 	Logger        *slog.Logger
@@ -52,6 +54,7 @@ func NewWorker(platform, callbackURL string, store EventStore, client *http.Clie
 		MaxRetries:    maxRetries,
 		BatchSize:     20,
 		PollInterval:  5 * time.Second,
+		ClaimTimeout:  5 * time.Minute,
 		RetrySchedule: []time.Duration{10 * time.Second, 30 * time.Second, 60 * time.Second, 5 * time.Minute, 15 * time.Minute},
 		Now:           time.Now,
 	}
@@ -63,6 +66,8 @@ func (w *Worker) Start(ctx context.Context) {
 	}
 	ticker := time.NewTicker(w.pollInterval())
 	defer ticker.Stop()
+	claimTicker := time.NewTicker(30 * time.Second)
+	defer claimTicker.Stop()
 	for {
 		select {
 		case <-ctx.Done():
@@ -77,6 +82,16 @@ func (w *Worker) Start(ctx context.Context) {
 			return
 		case <-ticker.C:
 		}
+		select {
+		case <-ctx.Done():
+			return
+		case <-claimTicker.C:
+			if w.Store != nil {
+				if _, err := w.Store.ReleaseStaleClaims(ctx, w.claimTimeout()); err != nil && w.Logger != nil {
+					w.Logger.Error("release stale claims failed", "platform", w.Platform, "error", err.Error())
+				}
+			}
+		}
 	}
 }
 
@@ -169,6 +184,13 @@ func (w *Worker) pollInterval() time.Duration {
 	return w.PollInterval
 }
 
+func (w *Worker) claimTimeout() time.Duration {
+	if w.ClaimTimeout <= 0 {
+		return 5 * time.Minute
+	}
+	return w.ClaimTimeout
+}
+
 func (w *Worker) now() time.Time {
 	if w.Now == nil {
 		return time.Now()