Fix media_player.play_media by using go2rtc /api/ffmpeg

[mayerwin]

Problem

media_player.play_media on platform: webrtc fails with HTTP 500 "can't find consumer" on many real setups (reported across multiple issues by Reolink / Amcrest / Dahua users trying to use the entity for TTS or alarm playback).

Root cause: the integration sends

POST /api/streams?dst=<stream>&src=ffmpeg:<url>#audio=<user_codec>#input=file

so the ffmpeg producer advertises one hardcoded codec (whatever the user put in audio:). If the camera's backchannel does not accept that exact codec — e.g. the user configures audio: pcma but the Reolink backchannel negotiates PCMU/8000 — go2rtc's matchMedia returns false and stream.Play returns "can't find consumer" at play.go#L108. The error message is misleading; its actual meaning is "no codec match between producer offer and backchannel consumer."

Fix

Switch async_play_media to go2rtc's /api/ffmpeg helper. Per ffmpeg/api.go#L27, this endpoint hardcodes #audio=auto on the ffmpeg producer, so the producer advertises the full codec menu (Opus 48k, L16/PCMA/PCMU at 16k then 8k, AAC) and go2rtc picks whichever the camera's backchannel actually supports.

Both endpoints ultimately call the same stream.Play(src) — /api/ffmpeg just builds src with audio=auto instead of passing through whatever the caller supplied, making it a strict superset for async_play_media's use case.

audio: config option is now Optional (kept for backwards compat so existing YAML still validates) but unused — codec selection is entirely auto-negotiated by go2rtc. The old #-fragment source string is gone, which incidentally removes some subtle URL-encoding risk for file= values that contain query params.

Diff summary

• custom_components/webrtc/media_player.py: schema audio Required → Optional; async_play_media uses utils.api_ffmpeg with params={dst, file}.
• custom_components/webrtc/utils.py: api_ffmpeg() helper mirroring api_streams().

Total: +15 / -10 lines.

Verified on

• Reolink camera with ONVIF source (port 8000), go2rtc 1.9.9. Before: consistent HTTP 500 "can't find consumer" on every codec value tried (pcma, pcma/8000, pcmu, pcmu/8000, opus, opus/48000/2, aac). After: media_player.play_media with TTS (Piper via HA media_source) and with a plain mp3 URL both succeed within ~5s, audio plays at the camera speaker.
• Inspection of go2rtc's producers/consumers state after a successful /api/ffmpeg call confirmed the newly-added consumer advertises audio, sendonly, PCMU/8000 — i.e. the camera's actual preferred codec, not the pcma we'd been forcing.

Compatibility

Existing configs with audio: pcma (or similar) continue to validate and work — the field is now simply ignored. No user-facing breaking change.