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.

.woodpecker/main-amd64-build.yml

@@ -36,6 +36,11 @@ steps:
       ZOT_USERNAME: justme
       ZOT_SECRET:
         from_secret: zot_secret
+      RELEASE_API_URL: https://mo.oeo.one
+      RELEASE_SIGNING_KEY:
+        from_secret: release_signing_key
+      RELEASE_API_TOKEN:
+        from_secret: release_api_token
     commands:
       - bash ci/build-main-amd64.sh agent
     depends_on: []
@@ -48,6 +53,11 @@ steps:
       ZOT_USERNAME: justme
       ZOT_SECRET:
         from_secret: zot_secret
+      RELEASE_API_URL: https://mo.oeo.one
+      RELEASE_SIGNING_KEY:
+        from_secret: release_signing_key
+      RELEASE_API_TOKEN:
+        from_secret: release_api_token
     commands:
       - bash ci/build-main-amd64.sh probe
     depends_on: []