BE-PR-010: deprecate feat_task_id — retain column, read-only compat

- Updated model docstring with full deprecation strategy
- Updated column comment to mark as deprecated (BE-PR-010)
- Updated schema/router comments for deprecation clarity
- Added deprecation doc: docs/BE-PR-010-feat-task-id-deprecation.md
- feat_task_id superseded by Task.source_proposal_id (BE-PR-008)

app/api/routers/proposals.py

@@ -51,7 +51,7 @@ def _serialize_proposal(db: Session, proposal: Proposal, *, include_essentials:
         "project_id": proposal.project_id,
         "created_by_id": proposal.created_by_id,
         "created_by_username": creator.username if creator else None,
-        "feat_task_id": proposal.feat_task_id,  # DEPRECATED — read-only compat
+        "feat_task_id": proposal.feat_task_id,  # DEPRECATED (BE-PR-010): read-only for legacy rows. Clients should use generated_tasks.
         "created_at": proposal.created_at,
         "updated_at": proposal.updated_at,
     }
@@ -237,7 +237,7 @@ def update_proposal(
         raise HTTPException(status_code=403, detail="Proposal edit permission denied")
 
     data = proposal_in.model_dump(exclude_unset=True)
-    # Never allow client to set feat_task_id
+    # DEPRECATED (BE-PR-010): feat_task_id is read-only; strip from client input
     data.pop("feat_task_id", None)
 
     for key, value in data.items():
@@ -365,7 +365,7 @@ def accept_proposal(
         })
         next_num = task.id + 1  # use real id for next code to stay consistent
 
-    # Update proposal status (do NOT write feat_task_id — deprecated)
+    # Update proposal status — feat_task_id is NOT written (deprecated per BE-PR-010)
     proposal.status = ProposalStatus.ACCEPTED
 
     db.commit()