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>

docs/deployment/environment.md

@@ -123,7 +123,7 @@ Controls the base domain for store subdomains and custom-domain features.
 
 | Variable | Description | Default | Required |
 |---|---|---|---|
-| `PLATFORM_DOMAIN` | Root domain under which store subdomains are created | `wizard.lu` | No |
+| `MAIN_DOMAIN` | Root domain under which store subdomains are created | `wizard.lu` | No |
 | `ALLOW_CUSTOM_DOMAINS` | Allow stores to use their own domain names | `True` | No |
 | `REQUIRE_DOMAIN_VERIFICATION` | Require DNS verification before activating a custom domain | `True` | No |
 | `SSL_PROVIDER` | SSL certificate provider (`letsencrypt`, `cloudflare`, `manual`) | `letsencrypt` | No |