feat: add GPG key setup script and generalize nginx GPG key serving

Add script/setup/gpg.sh to generate a dedicated lair keyring with a
certify-only master key and a 1-year signing subkey, cross-signed by
both personal keys. The public key is synced to oolon as <short-id>.gpg.

Update nginx config to serve any .gpg file instead of a hardcoded
RPM-GPG-KEY-mistralrs path, supporting multiple keys as the repo grows.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

asset/nginx/rpm.lair.cafe.conf

@@ -30,7 +30,7 @@ server {
         add_header Cache-Control "no-cache, must-revalidate";
     }
 
-    location = /RPM-GPG-KEY-mistralrs {
+    location ~ \.gpg$ {
         default_type text/plain;
     }
 }

script/setup/gpg.sh

@@ -0,0 +1,99 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+keyring_dir="${HOME}/.gnupg/lair"
+key_uid="rpm@lair.cafe"
+remote_host=oolon
+remote_key_dir="/var/www/rpm"
+signing_keys=(
+    "1C09AC24C113C7F080DD4AA5B3C5A958508A43F2"
+    "CF3E5AA5DAFD4A7FB69053E393977688ACF3510F"
+)
+
+# ensure the lair keyring directory exists
+install --directory --mode 700 "${keyring_dir}"
+
+# check for an existing valid key in the lair keyring
+existing_fpr=$(gpg --homedir "${keyring_dir}" --batch --with-colons --list-keys "${key_uid}" 2>/dev/null \
+    | awk -F: '/^fpr:/ { print $10; exit }') || true
+
+if [ -n "${existing_fpr}" ]; then
+    echo "found existing key: ${existing_fpr}"
+else
+    echo "no key found for ${key_uid} in ${keyring_dir}, generating..."
+
+    # create a certify-only master key
+    gpg --homedir "${keyring_dir}" --batch --gen-key <<KEYEOF
+%no-protection
+Key-Type: eddsa
+Key-Curve: ed25519
+Key-Usage: cert
+Name-Real: lair.cafe RPM signing
+Name-Email: ${key_uid}
+Expire-Date: 0
+%commit
+KEYEOF
+    existing_fpr=$(gpg --homedir "${keyring_dir}" --batch --with-colons --list-keys "${key_uid}" \
+        | awk -F: '/^fpr:/ { print $10; exit }')
+    echo "generated master key: ${existing_fpr}"
+
+    # add a dedicated signing subkey with 1-year expiry
+    gpg --homedir "${keyring_dir}" --batch --passphrase '' --quick-add-key \
+        "${existing_fpr}" ed25519 sign 1y
+    echo "added signing subkey to ${existing_fpr}"
+
+    # sign the lair key with each personal key from the default keyring
+    gpg --homedir "${keyring_dir}" --batch --armor --export "${existing_fpr}" | gpg --import
+    for signer in "${signing_keys[@]}"; do
+        gpg --batch --yes --local-user "${signer}" --sign-key "${existing_fpr}"
+        echo "signed lair key with ${signer}"
+    done
+    gpg --armor --export "${existing_fpr}" | gpg --homedir "${keyring_dir}" --import
+    echo "imported signatures back into lair keyring"
+fi
+
+short_id="${existing_fpr: -8}"
+short_id_lower=$(echo "${short_id}" | tr '[:upper:]' '[:lower:]')
+public_key_file="${short_id_lower}.gpg"
+
+echo ""
+echo "key fingerprint: ${existing_fpr}"
+echo "short id:        ${short_id}"
+echo "public key file: ${public_key_file}"
+
+# export the public key in ascii-armored format
+gpg --homedir "${keyring_dir}" --batch --armor --export "${existing_fpr}" > "/tmp/${public_key_file}"
+echo "exported public key to /tmp/${public_key_file}"
+
+# sync public key to the remote rpm repo root (will not overwrite due to unique filename)
+if rsync \
+    --archive \
+    --verbose \
+    --ignore-existing \
+    --rsync-path 'sudo rsync' \
+    --chown root:root \
+    --chmod F644 \
+    "/tmp/${public_key_file}" \
+    "${remote_host}:${remote_key_dir}/${public_key_file}"; then
+    echo "sync'd public key to ${remote_host}:${remote_key_dir}/${public_key_file}"
+else
+    echo "failed to sync public key to ${remote_host}:${remote_key_dir}/${public_key_file}"
+    exit 1
+fi
+
+rm "/tmp/${public_key_file}"
+
+echo ""
+signing_subkey_fpr=$(gpg --homedir "${keyring_dir}" --batch --with-colons --list-keys "${key_uid}" \
+    | awk -F: '/^fpr:/ { fpr=$10 } /^sub:/ { getfpr=1; next } getfpr && /^fpr:/ { print $10; exit }')
+
+echo "next steps:"
+echo "  1. add the following secrets to the gitea repo:"
+echo "     RPM_SIGNING_KEY     = output of: gpg --homedir ${keyring_dir} --armor --export-secret-subkeys ${signing_subkey_fpr}!"
+echo "     RPM_SIGNING_KEY_ID  = ${key_uid}"
+echo "  2. users can import the key with:"
+echo "     sudo rpm --import https://rpm.lair.cafe/${public_key_file}"
+echo ""
+echo "  the master key (certify-only, no expiry) stays on this workstation in ${keyring_dir}."
+echo "  the signing subkey (1-year expiry) is what CI uses. rotate it with:"
+echo "     gpg --homedir ${keyring_dir} --quick-add-key ${existing_fpr} ed25519 sign 1y"