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.

.github/workflows/release.yml

@@ -27,6 +27,28 @@ jobs:
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
           SENTRY_DSN: ${{ secrets.SENTRY_DSN }}
+          # Empty when RELEASE_SIGNING_PUBKEY variable is unset — goreleaser
+          # accepts it and the resulting binary disables signature checks
+          # (back-compat: pre-signing releases continue to update). Set
+          # RELEASE_SIGNING_PUBKEY (variable) + RELEASE_SIGNING_KEY (secret)
+          # to turn verification on.
+          RELEASE_SIGNING_PUBKEY: ${{ vars.RELEASE_SIGNING_PUBKEY }}
+
+      - name: Sign checksums.txt with ed25519
+        # Reference secrets.X directly — step-level env defined in this same
+        # step is unreliable to read from this step's own if: expression.
+        if: ${{ vars.RELEASE_SIGNING_PUBKEY != '' && secrets.RELEASE_SIGNING_KEY != '' }}
+        env:
+          RELEASE_SIGNING_KEY: ${{ secrets.RELEASE_SIGNING_KEY }}
+          RELEASE_TAG: ${{ github.ref_name }}
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: |
+          set -euo pipefail
+          go run ./scripts/sign-checksums \
+            -key "$RELEASE_SIGNING_KEY" \
+            -in  dist/checksums.txt \
+            -out dist/checksums.txt.sig
+          gh release upload "$RELEASE_TAG" dist/checksums.txt.sig --clobber
 
   docker:
     needs: release