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>

scripts/verify-server.sh

@@ -63,7 +63,7 @@ if [ "$MODE" = "dev" ]; then
         fail ".env file not found — copy from .env.example"
     fi
 
-    REQUIRED_KEYS="DATABASE_URL REDIS_URL JWT_SECRET_KEY ADMIN_EMAIL PLATFORM_DOMAIN"
+    REQUIRED_KEYS="DATABASE_URL REDIS_URL JWT_SECRET_KEY ADMIN_EMAIL MAIN_DOMAIN"
     for key in $REQUIRED_KEYS; do
         val=$(env_val "$key")
         if [ -n "$val" ]; then