feat: add inject start/stop commands for one-way audio injection

[nickakt]

Summary

• New inject start / inject stop NG protocol commands
• Mixes audio from a source participant into a destination participant's stream via the audio player, without a full offer/answer exchange
• Supports cross-call injection via source-call-id (calls are merged internally)
• inject subscription attribute persisted through Redis for HA failover
• Audio-only; matches media streams by media-id, index, then type fallback

Protocol

{
  "command": "inject start",
  "call-id": "main-call-id",
  "to-tag": "dst-tag",
  "source-call-id": "src-call-id",
  "source-tag": "src-tag"
}

{
  "command": "inject stop",
  "call-id": "main-call-id",
  "to-tag": "dst-tag",
  "source-call-id": "src-call-id",
  "source-tag": "src-tag"
}

Files changed

• daemon/call.c — monologue_inject_start / monologue_inject_stop implementation
• daemon/call_interfaces.c — NG command handlers
• daemon/control_ng.c — command dispatch
• daemon/redis.c — inject flag persistence
• include/ — declarations and opmode enum
• docs/ng_control_protocol.md — documentation