fix: jsonb_set 嵌套路径无法创建多层 key 的问题

PostgreSQL jsonb_set 在 create_if_missing=true 时无法一次性创建多层嵌套路径。
例如设置 {antigravity_quota_scopes,gemini_image} 时，如果 antigravity_quota_scopes 不存在，
jsonb_set 不会自动创建外层 key，导致更新静默失败（affected=1 但数据未变）。

修复方案：嵌套两次 jsonb_set，先确保外层 key 存在，再设置内层值。

影响范围：
- SetAntigravityQuotaScopeLimit: Antigravity 平台按模型 scope 限流
- SetModelRateLimit: Anthropic 平台 Sonnet 模型限流

backend/internal/repository/account_repo.go

@@ -809,12 +809,21 @@ func (r *accountRepository) SetAntigravityQuotaScopeLimit(ctx context.Context, i
 		return err
 	}
 
-	path := "{antigravity_quota_scopes," + string(scope) + "}"
+	scopeKey := string(scope)
 	client := clientFromContext(ctx, r.client)
 	result, err := client.ExecContext(
 		ctx,
-		"UPDATE accounts SET extra = jsonb_set(COALESCE(extra, '{}'::jsonb), $1::text[], $2::jsonb, true), updated_at = NOW(), last_used_at = NOW() WHERE id = $3 AND deleted_at IS NULL",
+		`UPDATE accounts SET 
-		path,
+			extra = jsonb_set(
+				jsonb_set(COALESCE(extra, '{}'::jsonb), '{antigravity_quota_scopes}'::text[], COALESCE(extra->'antigravity_quota_scopes', '{}'::jsonb), true),
+				ARRAY['antigravity_quota_scopes', $1]::text[],
+				$2::jsonb,
+				true
+			),
+			updated_at = NOW(),
+			last_used_at = NOW()
+		WHERE id = $3 AND deleted_at IS NULL`,
+		scopeKey,
 		raw,
 		id,
 	)
@@ -829,6 +838,7 @@ func (r *accountRepository) SetAntigravityQuotaScopeLimit(ctx context.Context, i
 	if affected == 0 {
 		return service.ErrAccountNotFound
 	}
+
 	if err := enqueueSchedulerOutbox(ctx, r.sql, service.SchedulerOutboxEventAccountChanged, &id, nil, nil); err != nil {
 		log.Printf("[SchedulerOutbox] enqueue quota scope failed: account=%d err=%v", id, err)
 	}
@@ -849,12 +859,19 @@ func (r *accountRepository) SetModelRateLimit(ctx context.Context, id int64, sco
 		return err
 	}
 
-	path := "{model_rate_limits," + scope + "}"
 	client := clientFromContext(ctx, r.client)
 	result, err := client.ExecContext(
 		ctx,
-		"UPDATE accounts SET extra = jsonb_set(COALESCE(extra, '{}'::jsonb), $1::text[], $2::jsonb, true), updated_at = NOW() WHERE id = $3 AND deleted_at IS NULL",
+		`UPDATE accounts SET 
-		path,
+			extra = jsonb_set(
+				jsonb_set(COALESCE(extra, '{}'::jsonb), '{model_rate_limits}'::text[], COALESCE(extra->'model_rate_limits', '{}'::jsonb), true),
+				ARRAY['model_rate_limits', $1]::text[],
+				$2::jsonb,
+				true
+			),
+			updated_at = NOW()
+		WHERE id = $3 AND deleted_at IS NULL`,
+		scope,
 		raw,
 		id,
 	)