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",
 ]

backend/app/models/project_transfer.py

@@ -0,0 +1,87 @@
+import uuid
+from datetime import datetime
+from decimal import Decimal
+
+from sqlalchemy import (
+    DateTime,
+    ForeignKey,
+    Numeric,
+    Text,
+    func,
+    text,
+)
+from sqlalchemy.dialects.postgresql import JSONB
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from app.db.base import Base
+from app.models.catalog import _timestamptz, _uuid_pk
+
+
+class ProjectTransfer(Base):
+    """project_transfers — Inter-project (inter-site) material transfer ledger.
+
+    Records the movement of materials from a source project warehouse to a
+    destination project. Stock is deducted from the source warehouse immediately
+    upon creation (status = 'in_transit') via out_transaction_id.
+    When the destination project PM confirms receipt in 'Arrivals', stock is
+    added to the destination warehouse (status = 'received').
+    While 'in_transit', the transfer can be edited or cancelled (restoring source stock).
+    Once 'received', the transfer is permanently locked from edit or cancellation.
+    """
+
+    __tablename__ = "project_transfers"
+    __mapper_args__ = {"eager_defaults": True}
+
+    id: Mapped[uuid.UUID] = _uuid_pk()
+    from_project_id: Mapped[uuid.UUID] = mapped_column(
+        ForeignKey("projects.id", ondelete="RESTRICT"), nullable=False
+    )
+    from_warehouse_id: Mapped[uuid.UUID] = mapped_column(
+        ForeignKey("warehouses.id", ondelete="RESTRICT"), nullable=False
+    )
+    to_project_id: Mapped[uuid.UUID] = mapped_column(
+        ForeignKey("projects.id", ondelete="RESTRICT"), nullable=False
+    )
+    material_id: Mapped[uuid.UUID] = mapped_column(
+        ForeignKey("materials.id", ondelete="RESTRICT"), nullable=False
+    )
+    brand_id: Mapped[uuid.UUID | None] = mapped_column(
+        ForeignKey("brands.id", ondelete="SET NULL")
+    )
+    unit_id: Mapped[int] = mapped_column(
+        ForeignKey("units.id", ondelete="RESTRICT"), nullable=False
+    )
+    quantity: Mapped[Decimal] = mapped_column(Numeric(18, 6), nullable=False)
+    specs: Mapped[dict] = mapped_column(JSONB, nullable=False, server_default=text("'{}'::jsonb"))
+    specs_hash: Mapped[str] = mapped_column(Text, nullable=False, server_default=text("'default'"))
+    note: Mapped[str | None] = mapped_column(Text)
+    status: Mapped[str] = mapped_column(Text, nullable=False, server_default=text("'in_transit'"))
+
+    out_transaction_id: Mapped[uuid.UUID | None] = mapped_column(
+        ForeignKey("inventory_transactions.id", ondelete="SET NULL")
+    )
+    transfer_date: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False, server_default=func.now()
+    )
+    transferred_by: Mapped[str | None] = mapped_column(Text)
+    received_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
+    received_by: Mapped[str | None] = mapped_column(Text)
+    to_warehouse_id: Mapped[uuid.UUID | None] = mapped_column(
+        ForeignKey("warehouses.id", ondelete="SET NULL")
+    )
+    receipt_allocations: Mapped[list | None] = mapped_column(JSONB)
+    created_at: Mapped[datetime] = _timestamptz()
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False, server_default=func.now(), onupdate=func.now()
+    )
+
+    from_project: Mapped["Project"] = relationship("Project", foreign_keys=[from_project_id], lazy="raise")
+    to_project: Mapped["Project"] = relationship("Project", foreign_keys=[to_project_id], lazy="raise")
+    from_warehouse: Mapped["Warehouse"] = relationship("Warehouse", foreign_keys=[from_warehouse_id], lazy="raise")
+    to_warehouse: Mapped["Warehouse | None"] = relationship("Warehouse", foreign_keys=[to_warehouse_id], lazy="raise")
+    material: Mapped["Material"] = relationship("Material", foreign_keys=[material_id], lazy="raise")
+    brand: Mapped["Brand | None"] = relationship("Brand", foreign_keys=[brand_id], lazy="raise")
+    unit: Mapped["Unit"] = relationship("Unit", foreign_keys=[unit_id], lazy="raise")
+    out_transaction: Mapped["InventoryTransaction | None"] = relationship(
+        "InventoryTransaction", foreign_keys=[out_transaction_id], lazy="raise"
+    )