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/webrtc_session_registry.go

@@ -1,106 +0,0 @@
-package cmd
-
-import (
-	"context"
-	"log"
-	"sync"
-
-	"github.com/torrentclaw/unarr/internal/config"
-	"github.com/torrentclaw/unarr/internal/engine"
-	"github.com/torrentclaw/unarr/internal/library/mediainfo"
-)
-
-// webrtcRegistry tracks per-session cancel funcs for active custom WebRTC
-// streams (engine.RunWebRTCStream goroutines). Each session lives only as
-// long as its DataChannel; the registry exists so duplicate sync responses
-// don't double-spawn the same session and so daemon shutdown can drain.
-var webrtcRegistry = &webrtcSessionRegistry{
-	cancels: make(map[string]context.CancelFunc),
-}
-
-type webrtcSessionRegistry struct {
-	mu      sync.Mutex
-	cancels map[string]context.CancelFunc
-}
-
-func (r *webrtcSessionRegistry) has(sessionID string) bool {
-	r.mu.Lock()
-	defer r.mu.Unlock()
-	_, ok := r.cancels[sessionID]
-	return ok
-}
-
-func (r *webrtcSessionRegistry) add(sessionID string, cancel context.CancelFunc) {
-	r.mu.Lock()
-	defer r.mu.Unlock()
-	r.cancels[sessionID] = cancel
-}
-
-func (r *webrtcSessionRegistry) remove(sessionID string) {
-	r.mu.Lock()
-	defer r.mu.Unlock()
-	delete(r.cancels, sessionID)
-}
-
-// cancelAllWebRTCSessions cancels every running session. Called on daemon
-// shutdown so pion peers and SSE consumers exit cleanly.
-func cancelAllWebRTCSessions() {
-	webrtcRegistry.mu.Lock()
-	cancels := make([]context.CancelFunc, 0, len(webrtcRegistry.cancels))
-	for _, c := range webrtcRegistry.cancels {
-		cancels = append(cancels, c)
-	}
-	webrtcRegistry.cancels = make(map[string]context.CancelFunc)
-	webrtcRegistry.mu.Unlock()
-	for _, c := range cancels {
-		c()
-	}
-}
-
-// stdLogger is a tiny adapter so engine.RunWebRTCStream can log through the
-// standard library logger without pulling in a logging dependency.
-type stdLogger struct{}
-
-func (stdLogger) Infof(format string, args ...any)  { log.Printf(format, args...) }
-func (stdLogger) Warnf(format string, args ...any)  { log.Printf("WARN: "+format, args...) }
-func (stdLogger) Errorf(format string, args ...any) { log.Printf("ERROR: "+format, args...) }
-
-// buildTranscodeRuntime resolves the ffmpeg/ffprobe binaries + config knobs
-// for the WebRTC streaming pipeline. Failure to resolve a binary returns a
-// runtime with empty paths so engine.RunWebRTCStream falls back to
-// passthrough — the user gets a clearer codec error from the browser than a
-// daemon-side abort.
-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,
-	}
-}