Implement WHIP server ingest

[Copilot]

Adds a WHIP server for receiving WebRTC streams (H.264 video + Opus audio) into the app, reusing the existing HttpServer infrastructure.

HTTP server enhancements (HttpServer.swift)

• Request body parsing via Content-Length
• Custom response Content-Type and headers
• 201 Created / 204 No Content status codes
• Prefix-based route matching for DELETE /whip/{id}

WebRTC receive path (WhipServerClient.swift)

• Accepts SDP offer via libdatachannel, returns SDP answer after ICE gathering
• rtcSetTrackCallback + rtcSetMessageCallback to receive RTP on incoming tracks
• H.264 RTP depacketization (single NAL + FU-A), SPS/PPS extraction → CMVideoFormatDescription → hardware decode via VideoDecoder
• Opus RTP → AVAudioConverter decode to PCM Int16 → CMSampleBuffer

WHIP protocol (WhipServer.swift)

• POST /whip — SDP offer/answer exchange, returns 201 with Location header
• DELETE /whip/{id} — client teardown
• Manages concurrent WhipServerClient instances keyed by UUID

Settings & model integration

• SettingsWhipServer / SettingsWhipServerStream in SettingsIngests.swift
• Database.whipServer with encode/decode
• ModelWhipServer.swift implements WhipServerDelegate, routes decoded video/audio into the buffered media pipeline via appendBufferedVideoSampleBuffer / appendBufferedAudioSampleBuffer
• whipCamera() / isWhipCameraOrMic() helpers in CommonUtils.swift

WHIP endpoint usage

A WHIP client (e.g. OBS, ffmpeg, GStreamer) sends an SDP offer:

POST /whip HTTP/1.1
Content-Type: application/sdp

v=0
...

Server responds with the SDP answer and a session URL for teardown:

HTTP/1.1 201 Created
Content-Type: application/sdp
Location: /whip/<session-uuid>

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