fix: start/stop for ring-only forwards + group sync via ring topology

Three issues fixed:

1. Start/stop returned 404 for forwards owned by other nodes:
   findLinkByTriple only checked local SQLite store. Added
   findLinkFromTopology fallback — startForward/stopForward now look up
   ring topology entries when the local store doesn't have the link.

2. Group labels didn't sync through the ring:
   handleForwardsAssign only updated local SQLite, never the ring
   topology. Added State.UpdateTopologyGroup + Engine.UpdateTopologyGroup
   — handleForwardsAssign now updates both. Added topologySync callback
   called after every e.state = tk.State (OnToken) and e.state = s
   (AdoptState) to re-apply local store group overrides onto the freshly
   adopted topology, so they survive state adoption and propagate via
   the next token forward. handleForwardsGroupDelete also clears ring
   topology entries.

3. Status-merge branch omitted Group field:
   ring-only forwards always showed '未分组'. Added Group: t.Link.Group
   to the topology-merge forwardStatus.

Verified: 4-node cluster, 5 forwards, group assigned on node-a
propagates to all nodes within one token cycle; stop from non-owning
node succeeds (HTTP 200) and worker stops on the owning node.

internal/cluster/ring.go

@@ -404,6 +404,20 @@ func (s *State) ForwardsOwnedBy(owner string) []*TopoEntry {
 	return out
 }
 
+// UpdateTopologyGroup sets the group label on the topology entry matching the
+// (local, remote, port) triple. Returns true if found. The updated entry
+// propagates to all nodes via the next token cycle — group changes sync
+// through the ring without a dedicated command.
+func (s *State) UpdateTopologyGroup(local, remote string, port int, group string) bool {
+	for _, e := range s.Topology {
+		if e.Local.Name == local && e.Remote.Name == remote && e.Link.RemotePort == port {
+			e.Link.Group = group
+			return true
+		}
+	}
+	return false
+}
+
 // OfflineReassign moves all active forwards owned by an offline node back into
 // PendingTasks (they become new tasks for the next lowest-load member). Returns
 // the reassigned task ids.