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>

.env.example

@@ -67,10 +67,10 @@ LOG_LEVEL=INFO
 LOG_FILE=logs/app.log
 
 # =============================================================================
-# PLATFORM DOMAIN CONFIGURATION
+# MAIN DOMAIN CONFIGURATION
 # =============================================================================
 # Your main platform domain
-PLATFORM_DOMAIN=wizard.lu
+MAIN_DOMAIN=wizard.lu
 
 # Custom domain features
 # Enable/disable custom domains