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>

tests/unit/middleware/test_platform_context.py

@@ -24,7 +24,7 @@ from sqlalchemy.orm import Session
 
 from app.core.frontend_detector import FrontendDetector
 from middleware.platform_context import (
-    DEFAULT_PLATFORM_CODE,
+    MAIN_PLATFORM_CODE,
     PlatformContextManager,
     PlatformContextMiddleware,
     get_current_platform,
@@ -174,7 +174,7 @@ class TestPlatformContextManager:
 
         assert context is not None
         assert context["detection_method"] == "default"
-        assert context["path_prefix"] == DEFAULT_PLATFORM_CODE
+        assert context["path_prefix"] == MAIN_PLATFORM_CODE
         assert context["clean_path"] == "/about"  # No path rewrite for main site
 
     def test_detect_default_platform_127_0_0_1(self):
@@ -187,7 +187,7 @@ class TestPlatformContextManager:
 
         assert context is not None
         assert context["detection_method"] == "default"
-        assert context["path_prefix"] == DEFAULT_PLATFORM_CODE
+        assert context["path_prefix"] == MAIN_PLATFORM_CODE
 
     def test_detect_default_platform_root_path(self):
         """Test default platform detection for root path."""
@@ -797,8 +797,8 @@ class TestHelperFunctions:
         assert "Platform not found" in exc_info.value.detail
 
     def test_default_platform_code_is_main(self):
-        """Test that DEFAULT_PLATFORM_CODE is 'main'."""
-        assert DEFAULT_PLATFORM_CODE == "main"
+        """Test that MAIN_PLATFORM_CODE is 'main'."""
+        assert MAIN_PLATFORM_CODE == "main"
 
 
 @pytest.mark.unit