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/schemas/schemas.py

@@ -18,7 +18,8 @@ class TaskTypeEnum(str, Enum):
 class TaskStatusEnum(str, Enum):
     OPEN = "open"
     PENDING = "pending"
-    PROGRESSING = "progressing"
+    UNDERGOING = "undergoing"
+    COMPLETED = "completed"
     CLOSED = "closed"
 
 
@@ -193,11 +194,19 @@ class ProjectMemberResponse(BaseModel):
         from_attributes = True
 
 
+class MilestoneStatusEnum(str, Enum):
+    OPEN = "open"
+    FREEZE = "freeze"
+    UNDERGOING = "undergoing"
+    COMPLETED = "completed"
+    CLOSED = "closed"
+
+
 # Milestone schemas
 class MilestoneBase(BaseModel):
     title: str
     description: Optional[str] = None
-    status: Optional[str] = "open"
+    status: Optional[MilestoneStatusEnum] = MilestoneStatusEnum.OPEN
     due_date: Optional[datetime] = None
     planned_release_date: Optional[datetime] = None
     depend_on_milestones: Optional[List[str]] = None
@@ -212,7 +221,7 @@ class MilestoneCreate(MilestoneBase):
 class MilestoneUpdate(BaseModel):
     title: Optional[str] = None
     description: Optional[str] = None
-    status: Optional[str] = None
+    status: Optional[MilestoneStatusEnum] = None
     due_date: Optional[datetime] = None
     planned_release_date: Optional[datetime] = None
     depend_on_milestones: Optional[List[str]] = None
@@ -223,6 +232,7 @@ class MilestoneResponse(MilestoneBase):
     id: int
     project_id: int
     created_by_id: Optional[int] = None
+    started_at: Optional[datetime] = None
     created_at: datetime
     updated_at: Optional[datetime] = None