feat(stream)!: retire WebRTC, HLS-only, bump 0.9.4

Drops the custom WebRTC DataChannel pipeline + pion deps + WSS signaling
client + wire framing. Every in-browser playback now uses HLS over HTTP
from the daemon (Tailscale/LAN/UPnP). Browser P2P never re-enabled.

Wire renames (incompatible with web < 2026-05-26): agent.WebRTCSession
=> agent.StreamSession, SyncResponse.WebRTCSessions (JSON: webrtcSessions)
=> StreamSessions (JSON: streamSessions). MIN_AGENT_VERSION is bumped
to 0.9.4 on the web side so older agents see an upgrade card.

Also fixes the libx264 'VBV bitrate > level limit' abort by clamping
the encoder bitrate to the effective output height instead of the
requested label (carried over from the prior 0.9.3 unreleased work).

The seed_file vertical (mode=seed_file handler + engine.SeedFile) was
retired with the in-browser P2P player. [downloads.webrtc] config block
deleted; existing TOML files with the section still parse fine.

internal/engine/probe.go

@@ -9,7 +9,7 @@ import (
 )
 
 // StreamProbe summarises the codec / container shape of a file as it relates
-// to the WebRTC streaming pipeline. It tells the transcoder whether bytes can
+// to the HLS streaming pipeline. It tells the transcoder whether bytes can
 // be streamed as-is, just remuxed to fragmented MP4, or fully transcoded.
 type StreamProbe struct {
 	// VideoCodec lowercased — e.g. "h264", "hevc", "av1", "vp9", "mpeg4".