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/models/__init__.py

@@ -32,6 +32,8 @@ from app.models.procurement import (
     TermsheetScheduleStep,
 )
 
+from app.models.project_transfer import ProjectTransfer
+
 __all__ = [
     "Base",
     "Unit",
@@ -60,4 +62,5 @@ __all__ = [
     "ItemDecision",
     "ItemReceipt",
     "QuoteAttachment",
+    "ProjectTransfer",
 ]