feat(audio): 课堂音频采集引入进程环回，与端点环回双源互补（ADR-001 Phase 0-3） - #9

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4, dorny/paths-filter@v3. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/