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/multi-tenant.md

@@ -113,26 +113,37 @@ platform.com/storefront/store3 → Store 3 Storefront
 
 ### Store Detection Logic
 
-The `StoreContextMiddleware` detects stores using this priority:
+The `StoreContextMiddleware` detects stores via `_detect_store_from_host_and_path()`.
+Before trying the three detection methods, two guards determine which methods apply:
+
+- **`is_platform_domain`**: `host == platform.domain` (e.g., `rewardflow.lu` itself) — skips Methods 1 & 2
+- **`is_subdomain_of_platform`**: host is a subdomain of `platform.domain` (e.g., `acme.rewardflow.lu`) — skips Method 1
 
 ```python
-def detect_store(request):
-    host = request.headers.get("host")
+def detect_store(host, path, platform):
+    platform_own_domain = platform.domain  # e.g. "rewardflow.lu"
+    is_platform_domain = (host == platform_own_domain)
+    is_subdomain_of_platform = (
+        host != platform_own_domain
+        and host.endswith(f".{platform_own_domain}")
+    )
 
-    # 1. Try custom domain first
-    store = find_by_custom_domain(host)
-    if store:
-        return store, "custom_domain"
+    # 1. Custom domain — skipped when host is platform domain or subdomain of it
+    if not is_platform_domain and not is_subdomain_of_platform:
+        main_domain = settings.main_domain  # e.g. "wizard.lu"
+        if host != main_domain and not host.endswith(f".{main_domain}"):
+            store = find_by_custom_domain(host)
+            if store:
+                return store, "custom_domain"
 
-    # 2. Try subdomain
-    if host != settings.platform_domain:
-        store_code = host.split('.')[0]
-        store = find_by_code(store_code)
+    # 2. Subdomain — skipped when host IS the platform domain
+    if not is_platform_domain and "." in host:
+        subdomain = host.split('.')[0]
+        store = find_by_code(subdomain)
         if store:
             return store, "subdomain"
 
-    # 3. Try path-based
-    path = request.url.path
+    # 3. Path-based — always runs as fallback
     if path.startswith("/store/") or path.startswith("/storefront/"):
         store_code = extract_code_from_path(path)
         store = find_by_code(store_code)