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/cmd/player_session_registry.go

@@ -0,0 +1,96 @@
+package cmd
+
+import (
+	"context"
+	"sync"
+
+	"github.com/torrentclaw/unarr/internal/config"
+	"github.com/torrentclaw/unarr/internal/engine"
+	"github.com/torrentclaw/unarr/internal/library/mediainfo"
+)
+
+// playerSessionRegistry tracks per-session cancel funcs for active in-browser
+// HLS streaming sessions. Each session lives only as long as its ffmpeg
+// process; the registry exists so duplicate sync responses don't double-spawn
+// the same session and so daemon shutdown can drain.
+var playerSessionRegistry = &playerSessionRegistryT{
+	cancels: make(map[string]context.CancelFunc),
+}
+
+type playerSessionRegistryT struct {
+	mu      sync.Mutex
+	cancels map[string]context.CancelFunc
+}
+
+func (r *playerSessionRegistryT) has(sessionID string) bool {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	_, ok := r.cancels[sessionID]
+	return ok
+}
+
+func (r *playerSessionRegistryT) add(sessionID string, cancel context.CancelFunc) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	r.cancels[sessionID] = cancel
+}
+
+func (r *playerSessionRegistryT) remove(sessionID string) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	delete(r.cancels, sessionID)
+}
+
+// cancelAllPlayerSessions cancels every running session. Called on daemon
+// shutdown so the ffmpeg children and SSE consumers exit cleanly.
+func cancelAllPlayerSessions() {
+	playerSessionRegistry.mu.Lock()
+	cancels := make([]context.CancelFunc, 0, len(playerSessionRegistry.cancels))
+	for _, c := range playerSessionRegistry.cancels {
+		cancels = append(cancels, c)
+	}
+	playerSessionRegistry.cancels = make(map[string]context.CancelFunc)
+	playerSessionRegistry.mu.Unlock()
+	for _, c := range cancels {
+		c()
+	}
+}
+
+// buildTranscodeRuntime resolves the ffmpeg/ffprobe binaries + config knobs
+// for the HLS streaming pipeline. Failure to resolve a binary returns a
+// runtime with empty paths so the caller can short-circuit instead of
+// launching a transcoder that will immediately fail.
+func buildTranscodeRuntime(ctx context.Context, cfg config.Config) engine.TranscodeRuntime {
+	if !cfg.Download.Transcode.Enabled {
+		return engine.TranscodeRuntime{Disabled: true}
+	}
+	ffmpegPath, errF := mediainfo.ResolveFFmpeg(cfg.Library.FFmpegPath)
+	ffprobePath, errP := mediainfo.ResolveFFprobe(cfg.Library.FFprobePath)
+	if errF != nil || errP != nil {
+		return engine.TranscodeRuntime{Disabled: true}
+	}
+	hw := engine.HWAccelNone
+	switch cfg.Download.Transcode.HWAccel {
+	case "auto":
+		hw = engine.DetectHWAccel(ctx, ffmpegPath)
+	case "nvenc":
+		hw = engine.HWAccelNVENC
+	case "qsv":
+		hw = engine.HWAccelQSV
+	case "vaapi":
+		hw = engine.HWAccelVAAPI
+	case "videotoolbox":
+		hw = engine.HWAccelVideoToolbox
+	case "none", "":
+		hw = engine.HWAccelNone
+	}
+	return engine.TranscodeRuntime{
+		FFmpegPath:   ffmpegPath,
+		FFprobePath:  ffprobePath,
+		HWAccel:      hw,
+		Preset:       cfg.Download.Transcode.Preset,
+		VideoBitrate: cfg.Download.Transcode.VideoBitrate,
+		AudioBitrate: cfg.Download.Transcode.AudioBitrate,
+		MaxHeight:    cfg.Download.Transcode.MaxHeight,
+	}
+}