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/db/init_db.py

@@ -432,3 +432,48 @@ async def ensure_audit_table(engine: AsyncEngine) -> None:
         for stmt in indexes:
             await conn.execute(text(stmt))
     logger.info("audit_log table ensured")
+
+
+async def ensure_project_transfers_table(engine: AsyncEngine) -> None:
+    """Create (if missing) the project_transfers table for inter-site transfers. Safe on any volume."""
+    ddl = """
+    CREATE TABLE IF NOT EXISTS project_transfers (
+        id                  UUID            NOT NULL DEFAULT gen_random_uuid() PRIMARY KEY,
+        from_project_id     UUID            NOT NULL REFERENCES projects(id) ON DELETE RESTRICT,
+        from_warehouse_id   UUID            NOT NULL REFERENCES warehouses(id) ON DELETE RESTRICT,
+        to_project_id       UUID            NOT NULL REFERENCES projects(id) ON DELETE RESTRICT,
+        material_id         UUID            NOT NULL REFERENCES materials(id) ON DELETE RESTRICT,
+        brand_id            UUID            REFERENCES brands(id) ON DELETE SET NULL,
+        unit_id             INTEGER         NOT NULL REFERENCES units(id) ON DELETE RESTRICT,
+        quantity            NUMERIC(18,6)   NOT NULL CHECK (quantity > 0),
+        specs               JSONB           NOT NULL DEFAULT '{}'::jsonb,
+        specs_hash          TEXT            NOT NULL DEFAULT 'default',
+        note                TEXT,
+        status              TEXT            NOT NULL DEFAULT 'in_transit' CHECK (status IN ('in_transit', 'received', 'cancelled')),
+        out_transaction_id  UUID            REFERENCES inventory_transactions(id) ON DELETE SET NULL,
+        transfer_date       TIMESTAMPTZ     NOT NULL DEFAULT now(),
+        transferred_by      TEXT,
+        received_at         TIMESTAMPTZ,
+        received_by         TEXT,
+        to_warehouse_id     UUID            REFERENCES warehouses(id) ON DELETE SET NULL,
+        receipt_allocations JSONB,
+        created_at          TIMESTAMPTZ     NOT NULL DEFAULT now(),
+        updated_at          TIMESTAMPTZ     NOT NULL DEFAULT now()
+    )
+    """
+    indexes = (
+        "CREATE INDEX IF NOT EXISTS idx_project_transfers_from ON project_transfers(from_project_id, status)",
+        "CREATE INDEX IF NOT EXISTS idx_project_transfers_to ON project_transfers(to_project_id, status)",
+        "DROP TRIGGER IF EXISTS trg_project_transfers_updated_at ON project_transfers",
+        """
+        CREATE TRIGGER trg_project_transfers_updated_at
+            BEFORE UPDATE ON project_transfers
+            FOR EACH ROW EXECUTE FUNCTION set_updated_at()
+        """,
+    )
+    async with engine.begin() as conn:
+        await conn.execute(text(ddl))
+        for stmt in indexes:
+            await conn.execute(text(stmt))
+    logger.info("project_transfers table ensured")
+