feat(loyalty): Phase 1 production launch hardening

Phase 1 of the loyalty production launch plan: config & security
hardening, dropped-data fix, DB integrity guards, rate limiting, and
constant-time auth compare. 362 tests pass.

- 1.4 Persist customer birth_date (new column + migration). Enrollment
  form was collecting it but the value was silently dropped because
  create_customer_for_enrollment never received it. Backfills existing
  customers without overwriting.
- 1.1 LOYALTY_GOOGLE_SERVICE_ACCOUNT_JSON validated at startup (file
  must exist and be readable; ~ expanded). Adds is_google_wallet_enabled
  and is_apple_wallet_enabled derived flags. Prod path documented as
  ~/apps/orion/google-wallet-sa.json.
- 1.5 CHECK constraints on loyalty_cards (points_balance, stamp_count
  non-negative) and loyalty_programs (min_purchase, points_per_euro,
  welcome_bonus non-negative; stamps_target >= 1). Mirrored as
  CheckConstraint in models. Pre-flight scan showed zero violations.
- 1.3 @rate_limit on store mutating endpoints: stamp 60/min,
  redeem/points-earn 30-60/min, void/adjust 20/min, pin unlock 10/min.
- 1.2 Constant-time hmac.compare_digest for Apple Wallet auth token
  (pulled forward from Phase 9 — code is safe whenever Apple ships).

See app/modules/loyalty/docs/production-launch-plan.md for the full
launch plan and remaining phases.

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

app/modules/loyalty/models/loyalty_card.py

@@ -19,6 +19,7 @@ from datetime import UTC, datetime
 
 from sqlalchemy import (
     Boolean,
+    CheckConstraint,
     Column,
     DateTime,
     ForeignKey,
@@ -259,6 +260,10 @@ class LoyaltyCard(Base, TimestampMixin, SoftDeleteMixin):
         Index("idx_loyalty_card_merchant_customer", "merchant_id", "customer_id", unique=True),
         Index("idx_loyalty_card_merchant_active", "merchant_id", "is_active"),
         Index("idx_loyalty_card_customer_program", "customer_id", "program_id", unique=True),
+        # Balances must never go negative — guards against direct SQL writes
+        # bypassing the service layer's clamping logic.
+        CheckConstraint("points_balance >= 0", name="ck_loyalty_cards_points_balance_nonneg"),
+        CheckConstraint("stamp_count >= 0", name="ck_loyalty_cards_stamp_count_nonneg"),
     )
 
     def __repr__(self) -> str: