BE-PR-002: Proposal model naming & field adjustments

- Add comprehensive docstring to Proposal model documenting all relationships
- Add column comments for all fields (title, description, status, project_id, etc.)
- Mark feat_task_id as DEPRECATED (will be replaced by Essential->task mapping in BE-PR-008)
- Add proposal_code hybrid property as preferred alias for DB column propose_code
- Update ProposalResponse schema to include proposal_code alongside propose_code
- Update serializer to emit both proposal_code and propose_code for backward compat
- No DB migration needed -- only Python-level changes

app/api/routers/proposals.py

@@ -24,16 +24,18 @@ router = APIRouter(prefix="/projects/{project_id}/proposals", tags=["Proposals"]
 def _serialize_proposal(db: Session, proposal: Proposal) -> dict:
     """Serialize proposal with created_by_username."""
     creator = db.query(models.User).filter(models.User.id == proposal.created_by_id).first() if proposal.created_by_id else None
+    code = proposal.propose_code  # DB column; also exposed as proposal_code
     return {
         "id": proposal.id,
         "title": proposal.title,
         "description": proposal.description,
-        "propose_code": proposal.propose_code,
+        "proposal_code": code,        # preferred name
+        "propose_code": code,         # backward compat
         "status": proposal.status.value if hasattr(proposal.status, "value") else proposal.status,
         "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,
+        "feat_task_id": proposal.feat_task_id,  # DEPRECATED — read-only compat
         "created_at": proposal.created_at,
         "updated_at": proposal.updated_at,
     }