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/middleware_test_routes.py

@@ -102,7 +102,7 @@ async def test_inactive_store_detection(request: Request):
 
 
 @router.get("/platform-domain")
-async def test_platform_domain(request: Request):
+async def test_main_domain(request: Request):
     """Test platform domain without subdomain."""
     store = getattr(request.state, "store", None)
     return {"store_detected": store is not None}