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>

app/modules/billing/routes/api/store_addons.py

@@ -144,7 +144,7 @@ def purchase_addon(
     store = billing_service.get_store(db, store_id)
 
     # Build URLs
-    base_url = f"https://{settings.platform_domain}"
+    base_url = f"https://{settings.main_domain}"
     success_url = f"{base_url}/store/{store.store_code}/billing?addon_success=true"
     cancel_url = f"{base_url}/store/{store.store_code}/billing?addon_cancelled=true"