♻️ refactor(model): replace gorm.io/datatypes with json.RawMessage for PrefillGroup.Items

- Why: Avoid adding `gorm.io/datatypes` for a single field; the rest of the codebase does not use it, and using the standard library keeps dependencies lean.
- What:
  - Switched `PrefillGroup.Items` from `datatypes.JSON` to `json.RawMessage`.
  - Updated imports in `model/prefill_group.go` to use `encoding/json` and removed the unused `gorm.io/datatypes`.
  - Preserved `gorm:"type:json"` so DB column behavior remains the same.
- Impact:
  - API response/request shape for `items` remains unchanged (still JSON).
  - DB schema behavior is unchanged; GORM migration continues to handle the field as JSON.
  - No other references to `datatypes` exist; no `go.mod` changes needed.
  - Lints pass for the modified file.

Files changed:
- model/prefill_group.go

No breaking changes.

model/prefill_group.go

@@ -1,9 +1,9 @@
 package model
 
 import (
+    "encoding/json"
     "one-api/common"
 
-    "gorm.io/datatypes"
     "gorm.io/gorm"
 )
 
@@ -18,7 +18,7 @@ type PrefillGroup struct {
     Id          int            `json:"id"`
     Name        string         `json:"name" gorm:"size:64;not null;uniqueIndex:uk_prefill_name,where:deleted_at IS NULL"`
     Type        string         `json:"type" gorm:"size:32;index;not null"`
-    Items       datatypes.JSON `json:"items" gorm:"type:json"`
+    Items       json.RawMessage `json:"items" gorm:"type:json"`
     Description string         `json:"description,omitempty" gorm:"type:varchar(255)"`
     CreatedTime int64          `json:"created_time" gorm:"bigint"`
     UpdatedTime int64          `json:"updated_time" gorm:"bigint"`