Draft: WebRTC clients

closes #56

this PR adds

• pipewire-rtp-sinks
• janus gateway
• count of wifi clients in mixer view
• a second frontend entry point at /index-client.html with following UI

ToDo

• auto hostname instead of hardcoded (frontend/src/client/janus.ts)
• secrets in config instead of hardcoded
• test ansible
• extensive testing
• set navigator.mediaSession.metadata
• show message when broadcasting is disabled

Later

• wifi setup (hardware + software) #89
• nginx config for public net #89

Screenshots