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/migrations/0002_remove_creditledger_billing_cre_team_id_e0f18f_idx_and_more.py

@@ -0,0 +1,30 @@
+# Generated by Django 5.1.15 on 2026-06-18 20:13
+
+from django.conf import settings
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('accounts', '0002_loginsession_userpreference'),
+        ('ai', '0008_aitask_ai_aitask_team_id_570138_idx'),
+        ('billing', '0001_initial'),
+        ('projects', '0003_scriptsegment_dialogue'),
+        migrations.swappable_dependency(settings.AUTH_USER_MODEL),
+    ]
+
+    operations = [
+        migrations.RemoveIndex(
+            model_name='creditledger',
+            name='billing_cre_team_id_e0f18f_idx',
+        ),
+        migrations.AddIndex(
+            model_name='creditledger',
+            index=models.Index(fields=['team', '-created_at'], name='billing_cre_team_id_c88290_idx'),
+        ),
+        migrations.AddIndex(
+            model_name='creditledger',
+            index=models.Index(fields=['team', 'ledger_type', 'created_at'], name='billing_cre_team_id_ffdcfe_idx'),
+        ),
+    ]