Created / Updated fields for clients (#3384)

* feat(backend): add created_at/updated_at to clients and maintain on create/update
backfill existing clients and set updated_at on mutations

* feat(frontend): carry created_at/updated_at in client models and round-trip via JSON

* feat(frontend): display Created and Updated columns in client table with proper date formatting

* i18n: add pages.inbounds.createdAt/updatedAt across all locales

* Update inbound.go

Remove duplicate code

database/model/model.go

@@ -104,4 +104,6 @@ type Client struct {
 	SubID      string `json:"subId" form:"subId"`
 	Comment    string `json:"comment" form:"comment"`
 	Reset      int    `json:"reset" form:"reset"`
+	CreatedAt  int64  `json:"created_at,omitempty"`
+	UpdatedAt  int64  `json:"updated_at,omitempty"`
 }