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/hls.go

@@ -3,9 +3,7 @@
 // Browser ↔ daemon over plain HTTP (LAN / Tailscale / UPnP). The daemon runs
 // ffmpeg in `-f hls` mode, writing fragmented MP4 segments to a per-session
 // tmpdir. Master + media playlists are pre-rendered from the probed source
-// duration so the player knows the full timeline before any segment exists,
-// which fixes the seek/duration/pause/multi-track problems we hit with the
-// raw fMP4-over-WebRTC pipeline.
+// duration so the player knows the full timeline before any segment exists.
 //
 // One HLSSession == one browser playback. Sessions are registered in a
 // process-wide map keyed by session ID; the StreamServer routes