Add AES67 streaming diagnostics and multicast loopback for debugging missing audio

[Copilot]

Debugging why audio doesn't arrive at an AES67 receiver was previously a blind spot — no visibility into packet flow and local testing required a second machine.

Changes

DiagStats — live stream counters

New exported struct tracked atomically from the streaming goroutine, reset on each session start:

• packets_sent, bytes_sent — confirm RTP traffic is flowing
• underruns — detect FFmpeg buffer starvation (→ silence injection)
• last_sent_unix — detect stalled sender
• stream_url, multicast_addr — confirm correct source/destination

GET /api/diag endpoint

Single call returns all counters + the exact SDP currently being advertised via SAP:

{
  "running": true,
  "station": "SRF 3",
  "multicast_addr": "239.0.0.1:5004",
  "stream_url": "https://stream.srg-ssr.ch/…",
  "packets_sent": 4250,
  "bytes_sent": 29155200,
  "underruns": 0,
  "last_sent_unix": 1744010255,
  "sdp": "v=0\r\no=- 0 0 IN IP4 …\r\n…"
}

Triage guide:

• packets_sent == 0 while running → FFmpeg decode failure
• packets_sent incrementing but receiver silent → network issue (IGMP, TTL, multicast routing)
• underruns > 0 → source stream is starving the RTP buffer

Multicast loopback

SetMulticastLoopback(true) on the sender socket so tools on the same host (ffplay, Wireshark, tcpdump) can receive the stream without a second machine — the necessary first step for local reproduction.

Original prompt

vorgehen zum testen weshalb audio auf aes67 emfänger nicht annkommt

Created from VS Code.

[silly82]

app laüft

[silly82]

aes test nr xxx