restore/undo for deleted messages — backend + UI

- POST /api/channels/{id}/messages/{msg_id}/restore — undeletes a message
- Any user can restore (not just the deleter)
- Broadcasts restored message via WebSocket
- UI: "undo" button in floating pill on deleted messages
- API: restoreMessage() in api.ts

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

ui/colony/src/components/MessageItem.tsx

@@ -11,6 +11,7 @@ interface Props {
   replyTarget?: Message;
   onSelect: (id: string) => void;
   onDelete: (channelId: string, msgId: string) => void;
+  onRestore: (channelId: string, msgId: string) => void;
   currentUsername: string;
   selected: boolean;
 }
@@ -72,7 +73,7 @@ function userHue(username: string): number {
   return Math.abs(hash) % 360;
 }
 
-export function MessageItem({ message, compact, replyTarget, onSelect, onDelete, currentUsername, selected }: Props) {
+export function MessageItem({ message, compact, replyTarget, onSelect, onDelete, onRestore, currentUsername, selected }: Props) {
   const [metaOpen, setMetaOpen] = useState(false);
   const isAgent = message.user.role === "agent";
   const isDeleted = !!message.deleted_at;
@@ -193,6 +194,16 @@ export function MessageItem({ message, compact, replyTarget, onSelect, onDelete,
               ×
             </button>
           )}
+          {isDeleted && (
+            <button
+              type="button"
+              onClick={(e) => { e.stopPropagation(); onRestore(message.channel_id, message.id); }}
+              className="px-2.5 py-1.5 text-[10px] font-mono text-muted-foreground hover:text-primary hover:bg-muted/50 transition-colors border-l-2 border-border"
+              title="Restore deleted message"
+            >
+              undo
+            </button>
+          )}
         </div>
 
         {/* Content */}