feat(inventory): implement inter-project material transfer backend

- Add ProjectTransfer database model and migration
- Implement project transfer service with immediate stock deduction on creation
- Support 100% full delivery receipt logic across destination warehouses
- Enforce canonical_specs_hash matching for cross-project stock merge
- Add RBAC guards and status lock on received transfers (409 Conflict)
- Register API router for transfers with on-the-way integration

backend/app/routers/__init__.py

@@ -14,6 +14,7 @@ from app.routers import (
     material_categories,
     materials,
     pm,
+    project_transfers,
     projects,
     quote_events,
     quotes,
@@ -50,5 +51,6 @@ ROUTERS = [
     standard_conversions.router,
     inventory.router,
     pm.router,
+    project_transfers.router,
     users.router,
 ]