docs: update documentation for platform-aware storefront routing

Update 8 documentation files to reflect new URL scheme:
- Dev: /platforms/{code}/storefront/{store_code}/
- Prod: subdomain.platform.lu/ (root path = storefront)
- Rename DEFAULT_PLATFORM_CODE to MAIN_PLATFORM_CODE
- Replace hardcoded platform_id=1 with dynamic values
- Update route examples, middleware descriptions, code samples

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

docs/architecture/metrics-provider-pattern.md

@@ -361,12 +361,12 @@ Platform context flows through middleware and JWT tokens:
 ┌─────────────────────────────────────────────────────────────────────┐
 │              Route Handler (Dashboard)                              │
 │                                                                      │
-│   # Get platform_id from middleware or JWT token                    │
-│   platform = getattr(request.state, "platform", None)               │
-│   platform_id = platform.id if platform else 1                      │
+│   # Get platform from require_platform dependency                   │
+│   platform = Depends(require_platform)  # Raises 400 if missing    │
+│   platform_id = platform.id                                         │
 │                                                                      │
 │   # Or from JWT for API routes                                      │
-│   platform_id = current_user.token_platform_id or 1                 │
+│   platform_id = current_user.token_platform_id                      │
 └─────────────────────────────────────────────────────────────────────┘
 ```
 
@@ -392,13 +392,11 @@ Platform context flows through middleware and JWT tokens:
 def get_store_dashboard_stats(
     request: Request,
     current_user: UserContext = Depends(get_current_store_api),
+    platform=Depends(require_platform),
     db: Session = Depends(get_db),
 ):
     store_id = current_user.token_store_id
-
-    # Get platform from middleware
-    platform = getattr(request.state, "platform", None)
-    platform_id = platform.id if platform else 1
+    platform_id = platform.id
 
     # Get aggregated metrics from all enabled modules
     metrics = stats_aggregator.get_store_dashboard_stats(