feat: use preset avatars and disable leaderboard by default

docs/refactor/performance-benchmark-runbook.md

@@ -61,6 +61,7 @@ npm run perf:api:local
 | `PERF_RAMP_SECONDS` | `3` | 并发爬坡时间 |
 | `PERF_QUESTION_LIMIT` | `20` | 单次合集题目请求数量 |
 | `PERF_INCLUDE_WRITES` | `false` | 是否加入创建练习 session 写请求 |
+| `PERF_INCLUDE_LEADERBOARD` | `false` | 是否加入排行榜接口；排行榜租户默认关闭，仅在租户明确开启并需要专项压测时打开 |
 | `PERF_OUTPUT_DIR` | `docs/refactor/performance-reports` | 报告输出目录 |
 | `PERF_REQUEST_TIMEOUT_MS` | `15000` | 单请求超时 |
 
@@ -78,12 +79,18 @@ npm run perf:api:local
 - `/api/catalog/practice-blueprints`
 - `/api/learning/stats`
 - `/api/learning/trend`
-- `/api/learning/leaderboard`
 - `/api/catalog/vocabulary-words`
 - `/api/learning/vocabulary/stats`
 - `/api/catalog/handbook-chapters`
 - `/api/catalog/handbook-entries?includeContent=true`
 
+排行榜租户默认关闭，不进入默认压测负载。只有在租户已经开启 `feature_flags.enableLeaderboard=true`，并准备验证独立排行榜页或活动页容量时，才打开：
+
+```powershell
+$env:PERF_INCLUDE_LEADERBOARD="true"
+npm run perf:api:local
+```
+
 写入路径默认关闭。只有在专门的测试库或可丢弃预生产库中，才建议打开：
 
 ```powershell