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/api/routers/proposals.py

@@ -63,6 +63,26 @@ def _serialize_proposal(db: Session, proposal: Proposal, *, include_essentials:
             .all()
         )
         result["essentials"] = [_serialize_essential(e) for e in essentials]
+
+        # BE-PR-008: include tasks generated from this Proposal via Accept
+        gen_tasks = (
+            db.query(Task)
+            .filter(Task.source_proposal_id == proposal.id)
+            .order_by(Task.id.asc())
+            .all()
+        )
+        result["generated_tasks"] = [
+            {
+                "task_id": t.id,
+                "task_code": t.task_code,
+                "task_type": t.task_type or "story",
+                "task_subtype": t.task_subtype,
+                "title": t.title,
+                "status": t.status.value if hasattr(t.status, "value") else t.status,
+                "source_essential_id": t.source_essential_id,
+            }
+            for t in gen_tasks
+        ]
     return result
 
 
@@ -327,6 +347,9 @@ def accept_proposal(
             reporter_id=proposal.created_by_id or current_user.id,
             created_by_id=proposal.created_by_id or current_user.id,
             task_code=task_code,
+            # BE-PR-008: track which Proposal/Essential generated this task
+            source_proposal_id=proposal.id,
+            source_essential_id=essential.id,
         )
         db.add(task)
         db.flush()  # materialise task.id