feat(procurement): support multi-project line item split allocation and merge on reopen

- Add ProjectAllocationIn and allocations field to ItemDecisionIn schema
- Implement line item split allocation across projects in accept_quote
- Replicate all ItemTermsheet options and LineItemSpec records to split rows
- Ensure QuoteProjects and LineItemProjects relations are updated
- Add validation against duplicate project selection and decimal sum mismatches
- Implement _merge_split_line_items_on_reopen to consolidate rows when reverting status to offered

backend/app/schemas/procurement.py

@@ -135,6 +135,11 @@ class AcceptTermsheetIn(APIModel):
     schedule_steps: list[ScheduleStepIn] = Field(min_length=1)
 
 
+class ProjectAllocationIn(APIModel):
+    project_id: UUID
+    quantity: Decimal = Field(gt=0, description="Allocated quantity for this project")
+
+
 class ItemDecisionIn(APIModel):
     line_item_id: UUID
     status: Literal["chosen", "rejected"]
@@ -142,6 +147,7 @@ class ItemDecisionIn(APIModel):
     adjusted_unit_price: Decimal | None = Field(default=None, gt=0)
     adjusted_notes: str | None = None
     project_id: UUID | None = Field(default=None, description="The ONE purchased project (v2.6 — required when the offer spans several projects)")
+    allocations: list[ProjectAllocationIn] = Field(default_factory=list, description="Multi-project split allocations for this line item")
 
 
 class ItemProjectsIn(APIModel):