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/init_production.py

@@ -390,14 +390,14 @@ def create_admin_settings(db: Session) -> int:
         },
         {
             "key": "platform_url",
-            "value": f"https://{settings.platform_domain}",
+            "value": f"https://{settings.main_domain}",
             "value_type": "string",
             "description": "Main platform URL",
             "is_public": True,
         },
         {
             "key": "support_email",
-            "value": f"support@{settings.platform_domain}",
+            "value": f"support@{settings.main_domain}",
             "value_type": "string",
             "description": "Platform support email",
             "is_public": True,