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/customers/migrations/versions/customers_003_add_birth_date.py

@@ -0,0 +1,31 @@
+"""customers 003 - add birth_date column
+
+Adds an optional birth_date column to the customers table so that
+self-enrollment flows (e.g. loyalty) can persist the customer's birthday
+collected on the enrollment form. Previously the field was collected by
+the UI and accepted by the loyalty service signature, but never written
+anywhere — see Phase 1.4 of the loyalty production launch plan.
+
+Revision ID: customers_003
+Revises: customers_002
+Create Date: 2026-04-09
+"""
+import sqlalchemy as sa
+
+from alembic import op
+
+revision = "customers_003"
+down_revision = "customers_002"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "customers",
+        sa.Column("birth_date", sa.Date(), nullable=True),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("customers", "birth_date")