The update operation for Model previously overwrote `created_time` with zero
because GORM included every struct field in the UPDATE statement.
This commit adjusts `Model.Update()` to:
* Call `Omit("created_time")` so the creation timestamp is never modified.
* Refresh `UpdatedTime` with `common.GetTimestamp()` before persisting.
* Delegate the remainder of the struct to GORM, eliminating the need to
maintain an explicit allow-list whenever new fields are introduced.
No API contract is changed; existing CRUD endpoints continue to work
normally while data integrity for historical records is now guaranteed.
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