feat: enrich member/comment/propose APIs with usernames

- ProjectMemberResponse now includes username and full_name
- Comment list endpoint returns author_username
- ProposeResponse now includes created_by_username
- All serializers resolve User objects to surface human-readable names
- Supports frontend code-first migration (TODO §3.1/3.2)

app/schemas/schemas.py

@@ -208,6 +208,8 @@ class ProjectMemberCreate(ProjectMemberBase):
 class ProjectMemberResponse(BaseModel):
     id: int
     user_id: int
+    username: Optional[str] = None
+    full_name: Optional[str] = None
     project_id: int
     role: str = "dev"
     
@@ -290,6 +292,7 @@ class ProposeResponse(ProposeBase):
     status: ProposeStatusEnum
     project_id: int
     created_by_id: Optional[int] = None
+    created_by_username: Optional[str] = None
     feat_task_id: Optional[str] = None
     created_at: datetime
     updated_at: Optional[datetime] = None