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>

scripts/seed/seed_demo.py

@@ -1445,7 +1445,7 @@ def print_summary(db: Session):
     print("\n🏪 Demo Stores:")
     for store in stores:
         print(f"\n  {store.name} ({store.store_code})")
-        print(f"    Subdomain: {store.subdomain}.{settings.platform_domain}")
+        print(f"    Subdomain: {store.subdomain}.{settings.main_domain}")
 
         # Show per-platform custom subdomains
         from app.modules.tenancy.models import Platform