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/tenancy/models/store.py

@@ -335,19 +335,19 @@ class Store(Base, TimestampMixin):
         Domain Resolution Priority:
         1. Store-specific custom domain (StoreDomain) -> highest priority
         2. Merchant domain (MerchantDomain) -> inherited default
-        3. Store subdomain ({store.subdomain}.{platform_domain}) -> fallback
+        3. Store subdomain ({store.subdomain}.{main_domain}) -> fallback
         """
         if self.primary_domain:
             return self.primary_domain
         if self.merchant and self.merchant.primary_domain:
             return self.merchant.primary_domain
-        return f"{self.subdomain}.{settings.platform_domain}"
+        return f"{self.subdomain}.{settings.main_domain}"
 
     @property
     def all_domains(self):
         """Get all active domains (subdomain + custom domains)."""
         domains = [
-            f"{self.subdomain}.{settings.platform_domain}"
+            f"{self.subdomain}.{settings.main_domain}"
         ]  # Start with the main subdomain
         for domain in self.domains:
             if domain.is_active: