feat: import/export settings, send backup of settings when leaving guild

Catalogger.Backend/Cache/InMemoryCache/InMemoryWebhookCache.cs

@@ -33,4 +33,11 @@ public class InMemoryWebhookCache : IWebhookCache
         _cache[channelId] = webhook;
         return Task.CompletedTask;
     }
+
+    public Task RemoveWebhooksAsync(ulong[] channelIds)
+    {
+        foreach (var id in channelIds)
+            _cache.TryRemove(id, out _);
+        return Task.CompletedTask;
+    }
 }