refactor: rename platform_domain → main_domain to avoid confusion with platform.domain

The setting `settings.platform_domain` (the global/main domain like "wizard.lu")
was easily confused with `platform.domain` (per-platform domain like "rewardflow.lu").
Renamed to `settings.main_domain` / `MAIN_DOMAIN` env var across the entire codebase.

Also updated docs to reflect the refactored store detection logic with
`is_platform_domain` / `is_subdomain_of_platform` guards.

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

tests/integration/middleware/conftest.py

@@ -49,7 +49,7 @@ def client(db):
 
     This patches:
     1. get_db in both middleware modules to use the test database
-    2. settings.platform_domain in store_context to use 'platform.com' for testing
+    2. settings.main_domain in store_context to use 'platform.com' for testing
 
     This ensures middleware can see test fixtures and detect subdomains correctly.
     """
@@ -64,7 +64,7 @@ def client(db):
 
     # Patch get_db in middleware modules - they have their own imports
     # The middleware calls: db_gen = get_db(); db = next(db_gen)
-    # Also patch settings.platform_domain so subdomain detection works with test hosts
+    # Also patch settings.main_domain so subdomain detection works with test hosts
     # Also bypass StorefrontAccessMiddleware subscription check for test routes —
     # these tests verify store context detection, not subscription access.
     from middleware.storefront_access import SKIP_PATH_PREFIXES
@@ -76,7 +76,7 @@ def client(db):
             with patch("middleware.theme_context.get_db", override_get_db):
                 with patch("middleware.store_context.settings") as mock_settings:
                     with patch("middleware.storefront_access.SKIP_PATH_PREFIXES", test_skip_prefixes):
-                        mock_settings.platform_domain = "platform.com"
+                        mock_settings.main_domain = "platform.com"
                         client = TestClient(app)
                         yield client