♻️ refactor(model): replace gorm.io/datatypes with JSONValue for PrefillGroup.Items; fix JSON scan across drivers

- Why:
  - Avoid introducing `gorm.io/datatypes` for a single field.
  - Align with existing pattern (`ChannelInfo`, `Properties`) using `Scanner`/`Valuer`.
  - Fix runtime error when drivers return JSON as string.

- What:
  - Introduced `JSONValue` (based on `json.RawMessage`) implementing `sql.Scanner` and `driver.Valuer`, with `MarshalJSON`/`UnmarshalJSON` to preserve raw JSON in API.
  - Updated `PrefillGroup.Items` to use `JSONValue` with `gorm:"type:json"`.
  - Localized comments in `model/prefill_group.go` to Chinese.

- Impact:
  - Resolves “unsupported Scan, storing driver.Value type string into type *json.RawMessage”.
  - Works with MySQL/Postgres/SQLite whether JSON is returned as `[]byte` or `string`.
  - API and DB schema remain unchanged; no `go.mod` changes; lints pass.

Files changed:
- model/prefill_group.go

web/src/components/table/models/modals/EditModelModal.jsx

@@ -390,7 +390,7 @@ const EditModelModal = (props) => {
                       editorType='object'
                       template={ENDPOINT_TEMPLATE}
                       templateLabel={t('填入模板')}
-                      extraText={(<Text type="tertiary" size="small">{t('留空则使用默认端点；支持 {path, method}')}</Text>)}
+                      extraText={t('留空则使用默认端点；支持 {path, method}')}
                       extraFooter={endpointGroups.length > 0 && (
                         <Space wrap>
                           {endpointGroups.map(group => (