perf(core): Wave 2 part1 — 复合索引(只生成不apply) + 消息中心 N+1 治理

索引(只 makemigrations 看 SQL,远程 MySQL apply 留用户低峰 / rule 3):
- Asset (team,category,-created_at)+(team,asset_type)、CreditLedger (team,-created_at)+
  (team,ledger_type,created_at)、Project (team,-updated_at)、AITask (team,-created_at)、
  Product (team,-created_at);迁移 assets0005/billing0002/projects0004/ai0008/products0002

消息中心 N+1(ops/views.py):
- type_counts 原 6 次 count(每请求)→ 一次 aggregate(Count(filter=...)) 条件聚合
- ensure_team_notifications 项目花费原逐项目 aggregate(N+1)→ 一次 values('project').annotate(Sum)

验证: apps.ops 测试 3/3 OK;全量 6fail+1err 均为预存 image provider 路由失败(stash 跑 baseline
  复现)= 零新增回归;索引迁移在 sqlite 测试库成功 apply

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

core/backend/apps/billing/models.py

@@ -40,7 +40,9 @@ class CreditLedger(TimeStampedModel):
 
     class Meta:
         indexes = [
-            models.Index(fields=["team", "ledger_type"]),
+            # 团队流水倒序翻页 + 按类型过滤(账户消费页 / 消息中心 charge 通知)
+            models.Index(fields=["team", "-created_at"]),
+            models.Index(fields=["team", "ledger_type", "created_at"]),
             models.Index(fields=["project", "task"]),
         ]