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/schemas/schemas.py

@@ -288,12 +288,13 @@ class ProposalUpdate(BaseModel):
 
 class ProposalResponse(ProposalBase):
     id: int
-    propose_code: Optional[str] = None  # DB column name kept for compat
+    proposal_code: Optional[str] = None  # preferred name
+    propose_code: Optional[str] = None   # backward compat alias (same value)
     status: ProposalStatusEnum
     project_id: int
     created_by_id: Optional[int] = None
     created_by_username: Optional[str] = None
-    feat_task_id: Optional[str] = None
+    feat_task_id: Optional[str] = None   # DEPRECATED — will be removed after BE-PR-008
     created_at: datetime
     updated_at: Optional[datetime] = None