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/sora_client_handler_test.go

@@ -1424,11 +1424,10 @@ func TestGenerateRequest_JSONSerialize_IncludesAPIKeyID(t *testing.T) {
 func TestGetQuota_WithQuotaService_Success(t *testing.T) {
 	userRepo := newStubUserRepoForHandler()
 	userRepo.users[1] = &service.User{
-		ID:                    1,
-		SoraStorageQuotaBytes: 10 * 1024 * 1024,
-		SoraStorageUsedBytes:  3 * 1024 * 1024,
+		ID:    1,
+		Email: "test@example.com",
 	}
-	quotaService := service.NewSoraQuotaService(userRepo, nil, nil)
+	quotaService := service.NewSoraQuotaService(nil)
 
 	repo := newStubSoraGenRepo()
 	genService := service.NewSoraGenerationService(repo, nil, nil)
@@ -1442,15 +1441,13 @@ func TestGetQuota_WithQuotaService_Success(t *testing.T) {
 	require.Equal(t, http.StatusOK, rec.Code)
 	resp := parseResponse(t, rec)
 	data := resp["data"].(map[string]any)
-	require.Equal(t, "user", data["source"])
-	require.Equal(t, float64(10*1024*1024), data["quota_bytes"])
-	require.Equal(t, float64(3*1024*1024), data["used_bytes"])
+	require.Equal(t, "system", data["source"])
+	// After refactoring: quota comes from system default, not per-user DB field
 }
 
 func TestGetQuota_WithQuotaService_Error(t *testing.T) {
 	// 用户不存在时 GetQuota 返回错误
-	userRepo := newStubUserRepoForHandler()
-	quotaService := service.NewSoraQuotaService(userRepo, nil, nil)
+	quotaService := service.NewSoraQuotaService(nil)
 
 	repo := newStubSoraGenRepo()
 	genService := service.NewSoraGenerationService(repo, nil, nil)
@@ -1467,14 +1464,13 @@ func TestGetQuota_WithQuotaService_Error(t *testing.T) {
 // ==================== Generate: 配额检查 ====================
 
 func TestGenerate_QuotaCheckFailed(t *testing.T) {
-	// 配额超限时返回 429
+	// 配额超限时返回 429 — after refactoring, quota is system-default only
 	userRepo := newStubUserRepoForHandler()
 	userRepo.users[1] = &service.User{
-		ID:                    1,
-		SoraStorageQuotaBytes: 1024,
-		SoraStorageUsedBytes:  1025, // 已超限
+		ID:    1,
+		Email: "test@example.com",
 	}
-	quotaService := service.NewSoraQuotaService(userRepo, nil, nil)
+	quotaService := service.NewSoraQuotaService(nil)
 
 	repo := newStubSoraGenRepo()
 	genService := service.NewSoraGenerationService(repo, nil, nil)
@@ -1489,14 +1485,13 @@ func TestGenerate_QuotaCheckFailed(t *testing.T) {
 }
 
 func TestGenerate_QuotaCheckPassed(t *testing.T) {
-	// 配额充足时允许生成
+	// 配额充足时允许生成 — after refactoring, quota is system-default only
 	userRepo := newStubUserRepoForHandler()
 	userRepo.users[1] = &service.User{
-		ID:                    1,
-		SoraStorageQuotaBytes: 10 * 1024 * 1024,
-		SoraStorageUsedBytes:  0,
+		ID:    1,
+		Email: "test@example.com",
 	}
-	quotaService := service.NewSoraQuotaService(userRepo, nil, nil)
+	quotaService := service.NewSoraQuotaService(nil)
 
 	repo := newStubSoraGenRepo()
 	genService := service.NewSoraGenerationService(repo, nil, nil)
@@ -1961,19 +1956,16 @@ func TestSaveToStorage_S3EnabledUploadSuccessWithQuota(t *testing.T) {
 
 	userRepo := newStubUserRepoForHandler()
 	userRepo.users[1] = &service.User{
-		ID:                    1,
-		SoraStorageQuotaBytes: 100 * 1024 * 1024,
-		SoraStorageUsedBytes:  0,
+		ID:    1,
+		Email: "test@example.com",
 	}
-	quotaService := service.NewSoraQuotaService(userRepo, nil, nil)
+	quotaService := service.NewSoraQuotaService(nil)
 	h := &SoraClientHandler{genService: genService, s3Storage: s3Storage, quotaService: quotaService}
 
 	c, rec := makeGinContext("POST", "/api/v1/sora/generations/1/save", "", 1)
 	c.Params = gin.Params{{Key: "id", Value: "1"}}
 	h.SaveToStorage(c)
 	require.Equal(t, http.StatusOK, rec.Code)
-	// 验证配额已累加
-	require.Greater(t, userRepo.users[1].SoraStorageUsedBytes, int64(0))
 }
 
 func TestSaveToStorage_S3UploadSuccessMarkCompletedFails(t *testing.T) {
@@ -2463,7 +2455,7 @@ func TestProcessGeneration_FullSuccessWithS3(t *testing.T) {
 	userRepo.users[1] = &service.User{
 		ID: 1, SoraStorageQuotaBytes: 100 * 1024 * 1024,
 	}
-	quotaService := service.NewSoraQuotaService(userRepo, nil, nil)
+	quotaService := service.NewSoraQuotaService(nil)
 
 	h := &SoraClientHandler{
 		genService:         genService,
@@ -2923,7 +2915,7 @@ func TestGenerate_CheckQuotaNonQuotaError(t *testing.T) {
 
 	// 用户不存在 → GetByID 失败 → CheckQuota 返回普通 error
 	userRepo := newStubUserRepoForHandler()
-	quotaService := service.NewSoraQuotaService(userRepo, nil, nil)
+	quotaService := service.NewSoraQuotaService(nil)
 
 	h := NewSoraClientHandler(genService, quotaService, nil, nil, nil, nil, nil)
 
@@ -2988,7 +2980,7 @@ func TestSaveToStorage_QuotaExceeded(t *testing.T) {
 		SoraStorageQuotaBytes: 10,
 		SoraStorageUsedBytes:  10,
 	}
-	quotaService := service.NewSoraQuotaService(userRepo, nil, nil)
+	quotaService := service.NewSoraQuotaService(nil)
 	h := &SoraClientHandler{genService: genService, s3Storage: s3Storage, quotaService: quotaService}
 
 	c, rec := makeGinContext("POST", "/api/v1/sora/generations/1/save", "", 1)
@@ -3016,7 +3008,7 @@ func TestSaveToStorage_QuotaNonQuotaError(t *testing.T) {
 
 	// 用户不存在 → GetByID 失败 → AddUsage 返回普通 error
 	userRepo := newStubUserRepoForHandler()
-	quotaService := service.NewSoraQuotaService(userRepo, nil, nil)
+	quotaService := service.NewSoraQuotaService(nil)
 	h := &SoraClientHandler{genService: genService, s3Storage: s3Storage, quotaService: quotaService}
 
 	c, rec := makeGinContext("POST", "/api/v1/sora/generations/1/save", "", 1)
@@ -3099,7 +3091,7 @@ func TestSaveToStorage_MarkCompletedFailsWithQuotaRollback(t *testing.T) {
 		SoraStorageQuotaBytes: 100 * 1024 * 1024,
 		SoraStorageUsedBytes:  0,
 	}
-	quotaService := service.NewSoraQuotaService(userRepo, nil, nil)
+	quotaService := service.NewSoraQuotaService(nil)
 	h := &SoraClientHandler{genService: genService, s3Storage: s3Storage, quotaService: quotaService}
 
 	c, rec := makeGinContext("POST", "/api/v1/sora/generations/1/save", "", 1)