feat: add Propose model/schema + DB enum migration scripts

- New Propose model (app/models/propose.py) with status enum (open/accepted/rejected)
- New Propose schemas (ProposeCreate/Update/Response) in schemas.py
- MySQL enum migration in main.py for milestone/task status columns
  - milestone: pending→open, deferred→closed, progressing→undergoing
  - task: progressing→undergoing
- Import propose model in startup for create_all
- Add started_at column migration for milestones

app/schemas/schemas.py

@@ -240,6 +240,42 @@ class MilestoneResponse(MilestoneBase):
         from_attributes = True
 
 
+# Propose schemas
+
+class ProposeStatusEnum(str, Enum):
+    OPEN = "open"
+    ACCEPTED = "accepted"
+    REJECTED = "rejected"
+
+
+class ProposeBase(BaseModel):
+    title: str
+    description: Optional[str] = None
+
+
+class ProposeCreate(ProposeBase):
+    project_id: Optional[int] = None
+
+
+class ProposeUpdate(BaseModel):
+    title: Optional[str] = None
+    description: Optional[str] = None
+
+
+class ProposeResponse(ProposeBase):
+    id: int
+    propose_code: Optional[str] = None
+    status: ProposeStatusEnum
+    project_id: int
+    created_by_id: Optional[int] = None
+    feat_task_id: Optional[str] = None
+    created_at: datetime
+    updated_at: Optional[datetime] = None
+
+    class Config:
+        from_attributes = True
+
+
 # Paginated response
 from typing import Generic, TypeVar
 T = TypeVar("T")