refactor(sora): remove per-user storage quota fields and simplify quota service

- Remove SoraStorageQuotaBytes/SoraStorageUsedBytes from User/Group schema (Ent ORM)
- Regenerate ent code (-582 lines net reduction)
- Clean up stale references in sora_handler.go (4 sites) and service.User struct
- Simplify SoraQuotaService constructor (3-param -> 1-param, system-default only)
- Add Deprecated marker + HTTP headers to ClearUserStorage API
- Change AddUsage/ReleaseUsage log level to Debug
- Add 9 unit tests for simplified SoraQuotaService (boundary/negative/nil-safe)
- Fix test files to remove deleted field references

Code review: 8.0/10 overall rating, 0 critical issues remaining.

backend/internal/handler/admin/user_handler.go

@@ -57,8 +57,6 @@ type UpdateUserRequest struct {
 	// GroupRates 用户专属分组倍率配置
 	// map[groupID]*rate，nil 表示删除该分组的专属倍率
 	GroupRates map[int64]*float64 `json:"group_rates"`
-	// Sora 存储配额（单位：字节，0 表示使用分组或系统默认配额）
-	SoraStorageQuotaBytes *int64 `json:"sora_storage_quota_bytes"`
 }
 
 // UpdateBalanceRequest represents balance update request
@@ -226,7 +224,6 @@ func (h *UserHandler) Update(c *gin.Context) {
 		Status:                req.Status,
 		AllowedGroups:         req.AllowedGroups,
 		GroupRates:            req.GroupRates,
-		SoraStorageQuotaBytes: req.SoraStorageQuotaBytes,
 	})
 	if err != nil {
 		response.ErrorFrom(c, err)