refactor: 统一使用 mergeMap 函数提升代码一致性

根据 Gemini CLI 代码审查建议：

## 修改内容
- 将 Gemini OAuth 同步中的 `mergeJSONB` 调用替换为 `mergeMap`
- 删除不再使用的 `mergeJSONB` 函数定义

## 原因
- 其他平台（OpenAI、Anthropic）的账户同步都使用 `mergeMap`
- `mergeJSONB` 是为旧的 `model.JSONB` 类型设计，与重构后的架构不一致
- 统一函数命名提高代码可读性和可维护性

## 影响范围
- backend/internal/service/crs_sync_service.go (4处替换)
- backend/internal/service/account.go (删除 mergeJSONB 函数)

## 验证
✓ 编译通过
✓ 功能逻辑无变化（mergeMap 和 mergeJSONB 实现相同）

backend/internal/service/account.go

@@ -326,17 +326,3 @@ func (a *Account) IsOpenAITokenExpired() bool {
 	}
 	return time.Now().Add(60 * time.Second).After(*expiresAt)
 }
-
-// mergeJSONB merges source map into target map (for preserving extra fields during account sync)
-func mergeJSONB(target, source map[string]any) map[string]any {
-	if target == nil {
-		target = make(map[string]any)
-	}
-	if source == nil {
-		return target
-	}
-	for k, v := range source {
-		target[k] = v
-	}
-	return target
-}

backend/internal/service/crs_sync_service.go

@@ -801,11 +801,11 @@ func (s *CRSSyncService) SyncFromCRS(ctx context.Context, input SyncFromCRSInput
 			continue
 		}
 
-		existing.Extra = mergeJSONB(existing.Extra, extra)
+		existing.Extra = mergeMap(existing.Extra, extra)
 		existing.Name = defaultName(src.Name, src.ID)
 		existing.Platform = PlatformGemini
 		existing.Type = AccountTypeOAuth
-		existing.Credentials = mergeJSONB(existing.Credentials, credentials)
+		existing.Credentials = mergeMap(existing.Credentials, credentials)
 		if proxyID != nil {
 			existing.ProxyID = proxyID
 		}
@@ -908,11 +908,11 @@ func (s *CRSSyncService) SyncFromCRS(ctx context.Context, input SyncFromCRSInput
 			continue
 		}
 
-		existing.Extra = mergeJSONB(existing.Extra, extra)
+		existing.Extra = mergeMap(existing.Extra, extra)
 		existing.Name = defaultName(src.Name, src.ID)
 		existing.Platform = PlatformGemini
 		existing.Type = AccountTypeApiKey
-		existing.Credentials = mergeJSONB(existing.Credentials, credentials)
+		existing.Credentials = mergeMap(existing.Credentials, credentials)
 		if proxyID != nil {
 			existing.ProxyID = proxyID
 		}