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/transcode_quality.go

@@ -0,0 +1,64 @@
+package engine
+
+// TranscodeRuntime carries the resolved ffmpeg/ffprobe paths + tunables so
+// each session can decide whether to passthrough or pipe through ffmpeg.
+type TranscodeRuntime struct {
+	FFmpegPath   string
+	FFprobePath  string
+	HWAccel      HWAccel
+	Preset       string
+	VideoBitrate string
+	AudioBitrate string
+	MaxHeight    int
+	// Disabled forces passthrough for every file even when codecs are not
+	// browser-friendly. Useful when the user explicitly turns transcoding
+	// off in config.
+	Disabled bool
+}
+
+// qualityCap maps a session's Quality label to a (MaxHeight, VideoBitrate)
+// pair. An empty label or "original" returns zero-values, signalling "no
+// override" to the caller.
+type qualityCap struct {
+	MaxHeight    int
+	VideoBitrate string // ffmpeg -b:v string, e.g. "3500k"
+}
+
+func resolveQualityCap(label string) qualityCap {
+	switch label {
+	case "2160p":
+		return qualityCap{MaxHeight: 2160, VideoBitrate: "25000k"}
+	case "1080p":
+		return qualityCap{MaxHeight: 1080, VideoBitrate: "6000k"}
+	case "720p":
+		return qualityCap{MaxHeight: 720, VideoBitrate: "3500k"}
+	case "480p":
+		return qualityCap{MaxHeight: 480, VideoBitrate: "1500k"}
+	default:
+		// "original", "auto", "" → defer to config.
+		return qualityCap{}
+	}
+}
+
+// capForHeight returns the bitrate-cap pair appropriate for an effective
+// output height. Used after clamping outputHeight to the source's resolution:
+// asking ffmpeg for "2160p" bitrate (25 Mbps) on a 1080p source overshoots
+// the H.264 level we derived from the EFFECTIVE height (4.0, max 20 Mbps) and
+// makes libx264 refuse with "VBV bitrate > level limit". This helper picks
+// the bitrate that matches the level libx264 will actually accept.
+func capForHeight(height int) qualityCap {
+	switch {
+	case height <= 0:
+		return qualityCap{}
+	case height <= 480:
+		return qualityCap{MaxHeight: 480, VideoBitrate: "1500k"}
+	case height <= 720:
+		return qualityCap{MaxHeight: 720, VideoBitrate: "3500k"}
+	case height <= 1080:
+		return qualityCap{MaxHeight: 1080, VideoBitrate: "6000k"}
+	case height <= 1440:
+		return qualityCap{MaxHeight: 1440, VideoBitrate: "12000k"}
+	default:
+		return qualityCap{MaxHeight: 2160, VideoBitrate: "25000k"}
+	}
+}