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/README.md

@@ -37,7 +37,7 @@ def client(db):
     with patch("middleware.store_context.get_db", override_get_db):
         with patch("middleware.theme_context.get_db", override_get_db):
             with patch("middleware.store_context.settings") as mock_settings:
-                mock_settings.platform_domain = "platform.com"
+                mock_settings.main_domain = "platform.com"
                 client = TestClient(app)
                 yield client
 ```