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/development/database-seeder/database-init-guide.md

@@ -911,7 +911,7 @@ settings.invitation_expiry_days       # int
 settings.database_url        # str
 
 # Platform
-settings.platform_domain     # str
+settings.main_domain         # str
 settings.project_name        # str
 ```