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>

.env.example

@@ -228,7 +228,10 @@ R2_BACKUP_BUCKET=orion-backups
 # See docs/deployment/hetzner-server-setup.md Step 25 for setup guide
 # Get Issuer ID from https://pay.google.com/business/console
 # LOYALTY_GOOGLE_ISSUER_ID=3388000000012345678
-# LOYALTY_GOOGLE_SERVICE_ACCOUNT_JSON=/path/to/service-account.json
+# Production convention: ~/apps/orion/google-wallet-sa.json (app user, mode 600).
+# Path is validated at startup — file must exist and be readable, otherwise
+# the app fails fast at import time.
+# LOYALTY_GOOGLE_SERVICE_ACCOUNT_JSON=~/apps/orion/google-wallet-sa.json
 # LOYALTY_GOOGLE_WALLET_ORIGINS=["https://yourdomain.com"]
 # LOYALTY_DEFAULT_LOGO_URL=https://yourdomain.com/path/to/default-logo.png