server: refuse unsigned releases and polluted reserved addresses

Self-update now verifies SHA256SUMS.sig (ed25519, relsign package) against
a public key baked into the binary; the private key exists only in the CI
secret store, so a compromised release host can withhold updates but not
inject one. CI signs on every server-v* tag and hard-fails without the
secret. Operators with their own pipeline override the key via
ECHOLOT_SELF_UPDATE_PUBKEY (mint a pair with release-sign -gen).

Startup also now proves 80/443 are actually free on the reserved
measurement addresses by asking the OS (throwaway bind), not the config -
CheckReserved could never see a stray process, and the adb-beacon receiver
on 0.0.0.0:443 was exactly that.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.gitea/workflows/build-server.yml

@@ -10,12 +10,18 @@
 # releases don't trigger each other's pipelines.
 #
 # Required secrets:
-#   REGISTRY_TOKEN  personal access token with read+write package scope —
-#                   the built-in Actions token is NOT accepted by the
-#                   container registry (docker login → unauthorized).
-#                   Create: user Settings → Applications → Generate token.
-#   REGISTRY_USER   optional; defaults to the pushing actor's username.
-# The release job needs only the built-in GITHUB_TOKEN.
+#   REGISTRY_TOKEN       personal access token with read+write package scope —
+#                        the built-in Actions token is NOT accepted by the
+#                        container registry (docker login → unauthorized).
+#                        Create: user Settings → Applications → Generate token.
+#   REGISTRY_USER        optional; defaults to the pushing actor's username.
+#   RELEASE_SIGNING_KEY  base64 ed25519 seed that signs SHA256SUMS. Self-updating
+#                        servers verify the signature against the public key baked
+#                        into the binary (selfupdate.DefaultPublicKeyB64) and REFUSE
+#                        unsigned releases, so this job hard-fails without it —
+#                        a release nobody can install is better failed loudly here.
+#                        Mint a pair with: go run ./cmd/release-sign -gen
+# The release job otherwise needs only the built-in GITHUB_TOKEN.
 
 name: server-release
 on:
@@ -44,6 +50,18 @@ jobs:
           done
           (cd ../dist && sha256sum * > SHA256SUMS)
 
+      - name: Sign SHA256SUMS
+        working-directory: server
+        env:
+          RELEASE_SIGNING_KEY: ${{ secrets.RELEASE_SIGNING_KEY }}
+        run: |
+          [ -n "$RELEASE_SIGNING_KEY" ] || { echo "::error::secret RELEASE_SIGNING_KEY is missing — self-updating servers refuse unsigned releases, so publishing one would strand the fleet. Add it under Settings → Actions → Secrets."; exit 1; }
+          go run ./cmd/release-sign ../dist/SHA256SUMS
+          # Verify with the key baked into the binary we just built — catches a
+          # secret that does not match DefaultPublicKeyB64 before it ships.
+          PUB=$(grep -o 'DefaultPublicKeyB64 = "[^"]*"' internal/selfupdate/selfupdate.go | cut -d'"' -f2)
+          go run ./cmd/release-sign -verify -pub "$PUB" ../dist/SHA256SUMS
+
       - name: Create release + attach binaries
         env:
           TOKEN: ${{ secrets.GITHUB_TOKEN }}