BE-PR-008: add Proposal Accept tracking fields (source_proposal_id, source_essential_id)

- Add source_proposal_id and source_essential_id FK columns to Task model
- Populate tracking fields during Proposal Accept task generation
- Add generated_tasks relationship on Proposal model for reverse lookup
- Expose source_proposal_id/source_essential_id in TaskResponse schema
- Add GeneratedTaskBrief schema and include generated_tasks in ProposalDetailResponse
- Proposal detail endpoint now returns generated story tasks with status

app/schemas/schemas.py

@@ -93,6 +93,9 @@ class TaskResponse(TaskBase):
     resolution_summary: Optional[str] = None
     positions: Optional[str] = None
     pending_matters: Optional[str] = None
+    # BE-PR-008: Proposal Accept tracking
+    source_proposal_id: Optional[int] = None
+    source_essential_id: Optional[int] = None
     created_at: datetime
     updated_at: Optional[datetime] = None
     
@@ -344,10 +347,22 @@ class EssentialResponse(EssentialBase):
         from_attributes = True
 
 
+class GeneratedTaskBrief(BaseModel):
+    """Brief info about a story task generated from Proposal Accept."""
+    task_id: int
+    task_code: Optional[str] = None
+    task_type: str
+    task_subtype: Optional[str] = None
+    title: str
+    status: Optional[str] = None
+    source_essential_id: Optional[int] = None
+
+
 class ProposalDetailResponse(ProposalResponse):
-    """Extended Proposal response that embeds its Essential list."""
+    """Extended Proposal response that embeds its Essential list and generated tasks."""
 
     essentials: List[EssentialResponse] = []
+    generated_tasks: List[GeneratedTaskBrief] = []
 
     class Config:
         from_attributes = True