refactor: update milestone/task status enums to new state machine values

Milestone: open/freeze/undergoing/completed/closed (was open/pending/deferred/progressing/closed)
Task: open/pending/undergoing/completed/closed (was open/pending/progressing/closed)

- Add MilestoneStatusEnum to schemas with typed validation
- Add started_at field to Milestone model
- Update all router/CLI references from progressing->undergoing
- Add completed status handling in task transition logic

app/api/routers/tasks.py

@@ -167,9 +167,9 @@ def update_task(task_id: int, task_update: schemas.TaskUpdate, db: Session = Dep
     update_data = task_update.model_dump(exclude_unset=True)
     if "status" in update_data:
         new_status = update_data["status"]
-        if new_status == "progressing" and not task.started_on:
+        if new_status == "undergoing" and not task.started_on:
             task.started_on = datetime.utcnow()
-        if new_status == "closed" and not task.finished_on:
+        if new_status in ("closed", "completed") and not task.finished_on:
             task.finished_on = datetime.utcnow()
     
     for field, value in update_data.items():
@@ -202,9 +202,9 @@ def transition_task(task_id: int, new_status: str, bg: BackgroundTasks, db: Sess
     if not task:
         raise HTTPException(status_code=404, detail="Task not found")
     old_status = task.status.value if hasattr(task.status, 'value') else task.status
-    if new_status == "progressing" and not task.started_on:
+    if new_status == "undergoing" and not task.started_on:
         task.started_on = datetime.utcnow()
-    if new_status == "closed" and not task.finished_on:
+    if new_status in ("closed", "completed") and not task.finished_on:
         task.finished_on = datetime.utcnow()
     task.status = new_status
     db.commit()