fix(security): UPnP opt-in, bounded SSE reader, signed self-update

Phase 2 security audit follow-up. Three independent hardenings against
the unauthenticated daemon surface, the long-lived agent SSE stream
and the self-update channel.

UPnP is now opt-in. The stream port + /hls endpoints have no auth, so
publishing them on the WAN via the gateway was a default that exposed
active downloads to anyone scanning the operator's external IP. New
config downloads.enable_upnp (default false) gates the mapping; LAN
and Tailscale clients continue to work unchanged. A startup log makes
the new default visible.

The agent SSE reader now uses a bounded bufio.Scanner instead of an
unbounded ReadString. A hostile or buggy server can no longer grow
daemon memory by streaming a single line forever or by emitting
unbounded data: continuation lines — both are capped at 256 KiB and
1 MiB respectively, and an error is surfaced so SignalLoop reconnects.

Self-update now verifies an ed25519 signature over checksums.txt when
the binary was built with a release public key embedded (injected via
goreleaser ldflags from RELEASE_SIGNING_PUBKEY). The companion
scripts/sign-checksums runs in the release workflow when both the
public-key variable and the private-key secret are present, uploading
checksums.txt.sig next to the existing checksums file. Builds without
the embedded key continue to update with SHA256-only verification; a
--allow-unsigned flag is provided so users on a signed build can
still install pre-signing releases or recover from an accidental
unsigned release.

A new scripts/gen-release-key helper documents the one-time keypair
generation procedure required before flipping signing on.

internal/cmd/daemon.go

@@ -240,6 +240,7 @@ func runDaemonStart() error {
 
 	// Create persistent stream server
 	streamSrv := engine.NewStreamServer(cfg.Download.StreamPort)
+	streamSrv.SetUPnPEnabled(cfg.Download.EnableUPnP)
 	// Reap HLS tmpdirs left over from a previous daemon run before we start
 	// accepting new sessions. The in-memory registry doesn't survive a
 	// restart, so without this disk usage grows unbounded across restarts.