docs: update routing docs and seed script for production routing changes

Reflect the production routing refactor (ce5b54f): document store dashboard
double-mounting, per-platform subdomain overrides via StorePlatform.custom_subdomain,
get_resolved_store_code dependency, and /merchants/ reserved path. Update seed
script to populate custom_subdomain and StoreDomain.platform_id for demo data.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/architecture/middleware.md

@@ -93,14 +93,18 @@ INFO Response: 200 for GET /admin/dashboard (0.143s)
 
 **What it does**:
 - Detects store from:
-  - Custom domain (e.g., `customdomain.com`)
-  - Subdomain (e.g., `store1.platform.com`)
+  - Custom domain (e.g., `customdomain.com`) — via `StoreDomain` lookup (optionally scoped to a platform via `StoreDomain.platform_id`)
+  - Subdomain with two-step lookup:
+    1. `StorePlatform.custom_subdomain` — per-platform subdomain overrides (e.g., `wizatech-rewards.rewardflow.lu`)
+    2. `Store.subdomain` — standard subdomain fallback (e.g., `wizatech.omsflow.lu`)
   - Path prefix (e.g., `/store/store1/` or `/stores/store1/`)
 - Queries database to find store by domain or code
-- Injects store object into `request.state.store`
+- Injects store object and platform into `request.state.store`
 - Extracts "clean path" (path without store prefix)
 - Sets `request.state.clean_path` for routing
 
+**Reserved paths**: When a storefront domain is detected, certain paths are excluded from the `/storefront/` rewrite: `/store/`, `/admin/`, `/api/`, `/static/`, `/storefront/`, `/health`, `/docs`, `/redoc`, `/media/`, `/assets/`, and `/merchants/`. This ensures that staff dashboards, APIs, and merchant-level routes are not incorrectly routed to storefront pages.
+
 **Example**:
 ```
 Request: https://orion.platform.com/storefront/products