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/request-flow.md

@@ -78,6 +78,7 @@ logger.info(f"Request: GET /storefront/products from 192.168.1.100")
 
 **What happens**:
 - Analyzes host header and path
+- Checks platform domain guards: if host matches `platform.domain`, Methods 1 & 2 are skipped; if host is a subdomain of `platform.domain`, Method 1 is skipped
 - Determines routing mode (custom domain / subdomain / path-based)
 - Queries database for store
 - Extracts clean path