fix: resolve remaining Inventory.location test failures and add Grafana config fields

- Fix test_inventory_service.py: replace model .location with .bin_location
- Fix test_product_model.py: remove location= from Inventory constructors
- Add grafana_admin_user/password to Settings for production Grafana config

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

app/core/config.py

@@ -199,6 +199,8 @@ class Settings(BaseSettings):
     # =============================================================================
     enable_metrics: bool = False
     grafana_url: str = "https://grafana.wizard.lu"
+    grafana_admin_user: str = "admin"
+    grafana_admin_password: str = ""
 
     # =============================================================================
     # CLOUDFLARE R2 STORAGE