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>

tests/integration/middleware/test_store_context_flow.py

@@ -7,7 +7,7 @@ for all routing modes: subdomain, custom domain, and path-based.
 
 Note: These tests require the middleware conftest.py which patches:
 1. get_db in middleware modules to use the test database session
-2. settings.platform_domain to use 'platform.com' for testing subdomain detection
+2. settings.main_domain to use 'platform.com' for testing subdomain detection
 """
 
 import pytest
@@ -137,7 +137,7 @@ class TestStoreContextFlow:
         data = response.json()
         assert data["store_detected"] is False
 
-    def test_platform_domain_without_subdomain_no_store(self, client):
+    def test_main_domain_without_subdomain_no_store(self, client):
         """Test that platform domain without subdomain doesn't detect store."""
         response = client.get(
             "/middleware-test/platform-domain", headers={"host": "platform.com"}