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/migrations/versions/loyalty_005_add_expiration_warning_at.py

@@ -0,0 +1,29 @@
+"""loyalty 005 - add last_expiration_warning_at to loyalty_cards
+
+Tracks when the last expiration warning email was sent to prevent
+duplicate notifications. The expiration task checks this timestamp
+and only sends a warning once per expiration cycle.
+
+Revision ID: loyalty_005
+Revises: loyalty_004
+Create Date: 2026-04-11
+"""
+import sqlalchemy as sa
+
+from alembic import op
+
+revision = "loyalty_005"
+down_revision = "loyalty_004"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "loyalty_cards",
+        sa.Column("last_expiration_warning_at", sa.DateTime(), nullable=True),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("loyalty_cards", "last_expiration_warning_at")