feat(loyalty): Phase 2A — transactional email notifications

Add async email notifications for 5 loyalty lifecycle events, using
the existing messaging module infrastructure (EmailService, EmailLog,
store template overrides).

- New seed script: scripts/seed/seed_email_templates_loyalty.py
  Seeds 5 templates × 4 locales (en/fr/de/lb) = 20 rows. Idempotent.
  Renamed existing script to seed_email_templates_core.py.
- Celery task: loyalty.send_notification_email — async dispatch with
  3 retries and 60s backoff. Opens own DB session.
- Notification service: LoyaltyNotificationService with 5 methods
  that resolve customer/card/program into template variables and
  enqueue via Celery (never blocks request handlers).
- Enrollment: sends loyalty_enrollment + loyalty_welcome_bonus (if
  bonus > 0) after card creation commit.
- Stamps: sends loyalty_reward_ready when stamp target reached.
- Expiration task: sends loyalty_points_expiring 14 days before expiry
  (tracked via new last_expiration_warning_at column to prevent dupes),
  and loyalty_points_expired after points are zeroed.
- Migration loyalty_005: adds last_expiration_warning_at to cards.
- 8 new unit tests for notification service dispatch.
- Fix: rate limiter autouse fixture in integration tests to prevent
  state bleed between tests.

Templates: loyalty_enrollment, loyalty_welcome_bonus,
loyalty_points_expiring, loyalty_points_expired, loyalty_reward_ready.
All support store-level overrides via the existing email template UI.

Birthday + re-engagement emails deferred to future marketing module
(cross-platform: OMS, loyalty, hosting).

342 tests pass.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/modules/loyalty/services/stamp_service.py

@@ -162,6 +162,22 @@ class StampService:
 
         wallet_service.sync_card_to_wallets(db, card)
 
+        # Notify customer when they've earned a reward
+        if reward_earned:
+            try:
+                from app.modules.loyalty.services.notification_service import (
+                    notification_service,
+                )
+
+                notification_service.send_reward_available(
+                    db, card, program.stamps_reward_description or "Reward"
+                )
+            except Exception:
+                logger.warning(
+                    f"Failed to queue reward notification for card {card.id}",
+                    exc_info=True,
+                )
+
         stamps_today += 1
 
         logger.info(