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>

app/core/config.py

@@ -6,7 +6,7 @@ This module provides classes and functions for:
 - Configuration management via environment variables
 - Database settings
 - JWT and authentication configuration
-- Platform domain and multi-tenancy settings
+- Main domain and multi-tenancy settings
 - Admin initialization settings
 
 Note: Environment detection is handled by app.core.environment module.
@@ -94,9 +94,9 @@ class Settings(BaseSettings):
     log_file: str | None = None
 
     # =============================================================================
-    # PLATFORM DOMAIN CONFIGURATION
+    # MAIN DOMAIN CONFIGURATION
     # =============================================================================
-    platform_domain: str = "wizard.lu"
+    main_domain: str = "wizard.lu"
 
     # Custom domain features
     allow_custom_domains: bool = True
@@ -353,7 +353,7 @@ def print_environment_info():
     print(f"  Database:     {settings.database_url}")
     print(f"  Debug mode:   {settings.debug}")
     print(f"  API port:     {settings.api_port}")
-    print(f"  Platform:     {settings.platform_domain}")
+    print(f"  Platform:     {settings.main_domain}")
     print(f"  Secure cookies: {should_use_secure_cookies()}")
     print("=" * 70 + "\n")