feat: signed releases and a host-side updater for container nodes

CI signs the image digest of each agent and probe release; the panel only
stores and forwards it. Nodes write the manifest to their data directory and
a host updater verifies the signature with a local public key, pulls by
digest, restarts the unit and rolls back when it is unhealthy. The panel can
list releases, ask a node to update and show the reported update state.

packaging/update/release-key.pub.pem

@@ -0,0 +1,4 @@
+-----BEGIN PUBLIC KEY-----
+MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEfw7EkNcl0FZqOETK+Vt55RMZ9K0q
+YOrtO3Vso/QNdSMmepQlx1Q3v+KkN7ro3AY3I6DT2Q2txJu5pS7qzutSQA==
+-----END PUBLIC KEY-----