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/backend/middleware-reference.md

@@ -43,9 +43,10 @@ The core authentication manager handling JWT tokens, password hashing, and role-
 Detects and manages store context from custom domains, subdomains, or path-based routing. This is the foundation of the multi-tenant system.
 
 **Key Features:**
+- Platform domain awareness: skips custom domain / subdomain detection when host is the platform's own domain
 - Custom domain routing (customdomain.com → Store)
 - Subdomain routing (store1.platform.com → Store)
-- Path-based routing (/store/store1/ → Store)
+- Path-based routing (/store/store1/ → Store) — always runs as fallback
 - Clean path extraction for nested routing
 
 ::: middleware.store_context.StoreContextManager