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/agent/signal_client_test.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"net/http"
 	"net/http/httptest"
+	"strings"
 	"sync"
 	"testing"
 	"time"
@@ -120,6 +121,48 @@ func TestSignalStreamCloseCancelsRead(t *testing.T) {
 	wg.Wait()
 }
 
+// TestSignalStreamRejectsOversizedEvent verifies that a hostile or buggy
+// server sending an unbounded `data:` event surfaces an error and stops
+// the reader instead of growing daemon memory forever.
+func TestSignalStreamRejectsOversizedEvent(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.Header.Get("Authorization") != "Bearer test-key" {
+			http.Error(w, "auth", http.StatusUnauthorized)
+			return
+		}
+		w.Header().Set("Content-Type", "text/event-stream")
+		flusher := w.(http.Flusher)
+		// Send many data: continuation lines until we blow past the
+		// per-event cap. Each chunk is a short legitimate-looking line.
+		chunk := "data: " + strings.Repeat("x", 4096) + "\n"
+		fmt.Fprint(w, "event: signal\n")
+		for i := 0; i < (sseMaxEventBytes/4096)+8; i++ {
+			fmt.Fprint(w, chunk)
+		}
+		flusher.Flush()
+		<-r.Context().Done()
+	}))
+	defer srv.Close()
+
+	c := NewClient(srv.URL, "test-key", "test-ua")
+	ctx, cancel := context.WithTimeout(context.Background(), 3*time.Second)
+	defer cancel()
+
+	stream, err := c.OpenSignalStream(ctx, "session-overflow")
+	if err != nil {
+		t.Fatalf("open: %v", err)
+	}
+	defer stream.Close()
+
+	for range stream.Events() {
+		// Should never receive a parsed event — the over-sized buffer must
+		// be rejected before dispatch.
+	}
+	if err := stream.Err(); err == nil {
+		t.Fatal("expected error from oversized event, got nil")
+	}
+}
+
 func TestPostSignalSendsCorrectBody(t *testing.T) {
 	var bodySeen map[string]any
 	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {