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.

go.mod

@@ -13,7 +13,6 @@ require (
 	github.com/google/uuid v1.6.0
 	github.com/huin/goupnp v1.3.0
 	github.com/olekukonko/tablewriter v1.1.4
-	github.com/pion/webrtc/v4 v4.2.11
 	github.com/spf13/cobra v1.10.2
 	github.com/torrentclaw/go-client v0.2.0
 	golang.org/x/term v0.43.0
@@ -107,6 +106,7 @@ require (
 	github.com/pion/stun/v3 v3.1.1 // indirect
 	github.com/pion/transport/v4 v4.0.1 // indirect
 	github.com/pion/turn/v4 v4.1.4 // indirect
+	github.com/pion/webrtc/v4 v4.2.11 // indirect
 	github.com/pkg/errors v0.9.1 // indirect
 	github.com/protolambda/ctxlock v0.1.0 // indirect
 	github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec // indirect