feat: platform-aware storefront routing and billing improvements

Overhaul storefront URL routing to be platform-aware:
- Dev: /platforms/{code}/storefront/{store_code}/
- Prod: subdomain.platform.lu/ (internally rewritten to /storefront/)
- Add subdomain detection in PlatformContextMiddleware
- Add /storefront/ path rewrite for prod mode (subdomain/custom domain)
- Remove all silent platform fallbacks (platform_id=1)
- Add require_platform dependency for clean endpoint validation
- Update route registration, templates, module definitions, base_url calc
- Update StoreContextMiddleware for /storefront/ path detection
- Remove /stores/ from FrontendDetector STOREFRONT_PATH_PREFIXES

Billing service improvements:
- Add store_platform_sync_service to keep store_platforms in sync
- Make tier lookups platform-aware across billing services
- Add tiers for all platforms in seed data
- Add demo subscriptions to seed

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/modules/billing/tests/unit/test_billing_service.py

@@ -603,7 +603,7 @@ class TestBillingServiceUpcomingInvoice:
 
 
 @pytest.fixture
-def bs_tier_essential(db):
+def bs_tier_essential(db, test_platform):
     """Create essential subscription tier."""
     tier = SubscriptionTier(
         code="essential",
@@ -614,6 +614,7 @@ def bs_tier_essential(db):
         display_order=1,
         is_active=True,
         is_public=True,
+        platform_id=test_platform.id,
     )
     db.add(tier)
     db.commit()
@@ -622,7 +623,7 @@ def bs_tier_essential(db):
 
 
 @pytest.fixture
-def bs_tiers(db):
+def bs_tiers(db, test_platform):
     """Create three tiers without Stripe config."""
     tiers = [
         SubscriptionTier(
@@ -633,6 +634,7 @@ def bs_tiers(db):
             display_order=1,
             is_active=True,
             is_public=True,
+            platform_id=test_platform.id,
         ),
         SubscriptionTier(
             code="professional",
@@ -642,6 +644,7 @@ def bs_tiers(db):
             display_order=2,
             is_active=True,
             is_public=True,
+            platform_id=test_platform.id,
         ),
         SubscriptionTier(
             code="business",
@@ -651,6 +654,7 @@ def bs_tiers(db):
             display_order=3,
             is_active=True,
             is_public=True,
+            platform_id=test_platform.id,
         ),
     ]
     db.add_all(tiers)
@@ -661,7 +665,7 @@ def bs_tiers(db):
 
 
 @pytest.fixture
-def bs_tiers_with_stripe(db):
+def bs_tiers_with_stripe(db, test_platform):
     """Create tiers with Stripe price IDs configured."""
     tiers = [
         SubscriptionTier(
@@ -672,6 +676,7 @@ def bs_tiers_with_stripe(db):
             display_order=1,
             is_active=True,
             is_public=True,
+            platform_id=test_platform.id,
             stripe_product_id="prod_essential",
             stripe_price_monthly_id="price_ess_monthly",
             stripe_price_annual_id="price_ess_annual",
@@ -684,6 +689,7 @@ def bs_tiers_with_stripe(db):
             display_order=2,
             is_active=True,
             is_public=True,
+            platform_id=test_platform.id,
             stripe_product_id="prod_professional",
             stripe_price_monthly_id="price_pro_monthly",
             stripe_price_annual_id="price_pro_annual",
@@ -696,6 +702,7 @@ def bs_tiers_with_stripe(db):
             display_order=3,
             is_active=True,
             is_public=True,
+            platform_id=test_platform.id,
             stripe_product_id="prod_business",
             stripe_price_monthly_id="price_biz_monthly",
             stripe_price_annual_id="price_biz_annual",