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/architecture/module-system.md

@@ -563,7 +563,7 @@ Every context includes these base variables regardless of modules:
 | `request` | FastAPI Request object |
 | `platform` | Platform model (may be None) |
 | `platform_name` | From settings.project_name |
-| `platform_domain` | From settings.platform_domain |
+| `main_domain` | From settings.main_domain |
 | `_` | Translation function (gettext style) |
 | `t` | Translation function (key-value style) |
 | `current_language` | Current language code |