fix(pipeline): throttle skipped L2 runs

[RerankerGuo]

Description | 描述

Throttle skipped L2 runs so an empty L2 query still updates the per-session L2 run timestamp and participates in l2MinIntervalSeconds. This prevents repeated cold-start/empty L2 cycles from immediately querying SQLite again across active sessions.

Also preserves the existing cursor on zero-record L2 runner skips and lowers the per-query SQLite L1 summary log from info to debug to reduce idle-period log noise.

Related Issue | 关联 Issue

Fix #155

Change Type | 修改类型

• Bug fix | Bug 修复
• New feature | 新功能
• Documentation update | 文档更新
• Code optimization | 代码优化

Self-test Checklist | 自测清单

• Verified locally | 本地验证通过
• No existing features affected | 无影响现有功能

Additional Notes | 其他说明

Verified with npx vitest run src/utils/pipeline-manager.test.ts, npm test, and npm run build using Node v24.15.0.

[Maxwell-Code07]

Throttling skipped L2 runs is a sensible optimization. Updating the timestamp even on skip prevents busy-looping — clean fix. We'll review it shortly.