feat: add /meta command for channel metadata retrieval

- Implemented the `/meta` command to fetch and display channel metadata including room ID, topic, message count, unique user count, estimated size, and protection level.
- Added `ChannelMetaDto` to encapsulate channel metadata.
- Updated `ChannelsController` to handle the new `/meta` endpoint.
- Introduced `UploadLimits` configuration for admin-defined upload size limits for files, images, audio, and avatars.
- Enhanced error handling and user feedback for metadata retrieval.
- Updated documentation to reflect changes in encryption and room metadata.
- Added tests for the new functionality and upload limits.

src/EchoHub.Client/Services/ApiClient.cs

@@ -271,6 +271,20 @@ public sealed class ApiClient : IDisposable
         return await response.Content.ReadFromJsonAsync<ChannelCryptoDto>();
     }
 
+    /// <summary>
+    /// Fetches a channel's human-facing metadata (message count, unique posters, estimated
+    /// size, created date, room id) for the <c>/meta</c> command. Returns null if it doesn't exist.
+    /// </summary>
+    public async Task<ChannelMetaDto?> GetChannelMetaAsync(string channelName)
+    {
+        EnsureAuthenticated();
+        using var response = await AuthenticatedGetAsync($"/api/channels/{Uri.EscapeDataString(channelName)}/meta");
+        if (response.StatusCode == System.Net.HttpStatusCode.NotFound)
+            return null;
+        await EnsureSuccessAsync(response);
+        return await response.Content.ReadFromJsonAsync<ChannelMetaDto>();
+    }
+
     public async Task<ChannelDto?> RekeyChannelAsync(string channelName, RekeyChannelRequest request)
     {
         EnsureAuthenticated();