fix(loyalty): cross-store enrollment, card scoping, i18n flicker

Fix duplicate card creation when the same email enrolls at different
stores under the same merchant, and implement cross-location-aware
enrollment behavior.

- Cross-location enabled (default): one card per customer per merchant.
  Re-enrolling at another store returns the existing card with a
  "works at all our locations" message + store list.
- Cross-location disabled: one card per customer per store. Enrolling
  at a different store creates a separate card for that store.

Changes:
- Migration loyalty_004: replace (merchant_id, customer_id) unique
  index with (enrolled_at_store_id, customer_id). Per-merchant
  uniqueness enforced at application layer when cross-location enabled.
- card_service.resolve_customer_id: cross-store email lookup via
  merchant_id param to find existing cardholders at other stores.
- card_service.enroll_customer: branch duplicate check on
  allow_cross_location_redemption setting.
- card_service.search_card_for_store: cross-store email search when
  cross-location enabled so staff at store2 can find cards from store1.
- card_service.get_card_by_customer_and_store: new service method.
- storefront enrollment: catch LoyaltyCardAlreadyExistsException,
  return existing card with already_enrolled flag, locations, and
  cross-location context. Server-rendered i18n via Jinja2 tojson.
- enroll-success.html: conditional cross-store/single-store messaging,
  server-rendered translations and context, i18n_modules block added.
- dashboard.html, history.html: replace $t() with server-side _() to
  fix i18n flicker across all storefront templates.
- Fix device-mobile icon → phone icon.
- 4 new i18n keys in 4 locales (en, fr, de, lb).
- Docs: updated data-model, business-logic, production-launch-plan,
  user-journeys with cross-location behavior and E2E test checklist.
- 12 new unit tests + 3 new integration tests (334 total pass).

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

app/modules/loyalty/migrations/versions/loyalty_004_relax_card_uniqueness.py

@@ -0,0 +1,59 @@
+"""loyalty 004 - relax card uniqueness for cross-location support
+
+Replace the (merchant_id, customer_id) and (customer_id, program_id)
+unique indexes with (enrolled_at_store_id, customer_id). This allows
+merchants with cross-location redemption DISABLED to issue one card per
+store per customer, while merchants with it ENABLED enforce the
+per-merchant constraint in the application layer.
+
+Revision ID: loyalty_004
+Revises: loyalty_003
+Create Date: 2026-04-10
+"""
+from alembic import op
+
+revision = "loyalty_004"
+down_revision = "loyalty_003"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    # Drop the old per-merchant unique indexes
+    op.drop_index("idx_loyalty_card_merchant_customer", table_name="loyalty_cards")
+    op.drop_index("idx_loyalty_card_customer_program", table_name="loyalty_cards")
+
+    # Keep a non-unique index on (merchant_id, customer_id) for lookups
+    op.create_index(
+        "idx_loyalty_card_merchant_customer",
+        "loyalty_cards",
+        ["merchant_id", "customer_id"],
+        unique=False,
+    )
+
+    # New unique constraint: one card per customer per store (always valid)
+    op.create_index(
+        "idx_loyalty_card_store_customer",
+        "loyalty_cards",
+        ["enrolled_at_store_id", "customer_id"],
+        unique=True,
+    )
+
+
+def downgrade() -> None:
+    op.drop_index("idx_loyalty_card_store_customer", table_name="loyalty_cards")
+    op.drop_index("idx_loyalty_card_merchant_customer", table_name="loyalty_cards")
+
+    # Restore original unique indexes
+    op.create_index(
+        "idx_loyalty_card_merchant_customer",
+        "loyalty_cards",
+        ["merchant_id", "customer_id"],
+        unique=True,
+    )
+    op.create_index(
+        "idx_loyalty_card_customer_program",
+        "loyalty_cards",
+        ["customer_id", "program_id"],
+        unique=True,
+    )