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/assets/models.py

@@ -45,6 +45,13 @@ class Asset(TeamOwnedModel):
     review_remote_id = models.CharField(max_length=128, blank=True, db_default="")
     review_error = models.TextField(blank=True, null=True, default="")  # TEXT 不能有 DB 默认值(MySQL 1101),改用可空容忍「漏带字段」的插入
 
+    class Meta:
+        indexes = [
+            # 资产库按团队 + 分类倒序时间翻页(products 详情素材趴 / 资产库)
+            models.Index(fields=["team", "category", "-created_at"]),
+            models.Index(fields=["team", "asset_type"]),
+        ]
+
     def __str__(self) -> str:
         return self.name