feat: install nodes and the backend from a static compose file; remove dead code [CI]

Agent, Probe and Backend now start from one static compose.yaml plus a local
.env: the panel (or the release assets, for the very first Backend) provides
the file, the panel's command writes the public values into .env, checks the
file's SHA-256 and runs compose up. Updating is compose pull && compose up -d.
The image is chosen by the CI channel (:latest, or MOSHIRO_VERSION in .env),
not by the panel; the previous one-line script stays as a fallback.

- packaging/compose/{agent,probe,backend}/compose.yaml are the single source,
  embedded in the Backend and served at /install/compose/{file}; the root
  compose.yaml moved there. Releases also carry them and list them in the
  signed SHA256SUMS.
- The container entrypoint defaults the node name to the host name.
- Join token creation returns a compose block (.env, url, hash, per-platform
  commands); the admin dialog leads with it.
- Remove code with no references (HotLayer members, LiveHub.SubscriberCount,
  QueryService.ParseParams, ReleaseService.Forget, a few constants) and reduce
  the install-code lookup to the role it is for.

backend/src/MoShiro.Backend.Application/Live/HotLayer.cs

@@ -28,8 +28,6 @@ public sealed class HotLayer
 
     public void MarkReady() => _ready = true;
 
-    public IEnumerable<(NodeId Agent, string Series)> Keys => _rings.Keys;
-
     /// <summary>评估一个块并合并进热层。同一个块重复合并（例如回填与实时竞争）会被忽略。</summary>
     public void Merge(NodeId agent, string series, RawChunk chunk)
     {
@@ -94,27 +92,6 @@ public sealed class HotLayer
         return result;
     }
 
-    public int BucketCount(NodeId agent, string series)
-    {
-        if (!_rings.TryGetValue((agent, series), out Ring? ring))
-        {
-            return 0;
-        }
-
-        lock (ring)
-        {
-            return ring.Buckets.Count;
-        }
-    }
-
-    public void Remove(NodeId agent)
-    {
-        foreach ((NodeId Agent, string Series) key in _rings.Keys.Where(k => k.Agent == agent).ToList())
-        {
-            _rings.TryRemove(key, out _);
-        }
-    }
-
     private static void Prune(Ring ring, long cutoff)
     {
         while (ring.Buckets.Count > 0)

backend/src/MoShiro.Backend.Application/Live/LiveHub.cs

@@ -45,8 +45,6 @@ public sealed class LiveHub
 {
     private readonly ConcurrentDictionary<LiveSubscription, byte> _subscriptions = new();
 
-    public int SubscriberCount => _subscriptions.Count;
-
     public LiveSubscription Subscribe()
     {
         var subscription = new LiveSubscription(this);