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.Tests/Irc/TestHelpers.cs

@@ -270,6 +270,11 @@ internal sealed class FakeChannelService : IChannelService
     public Task<ChannelDto?> GetChannelByNameAsync(string channelName) =>
         Task.FromResult(ChannelByNameToReturn);
 
+    public ChannelMetaDto? ChannelMetaToReturn { get; set; }
+
+    public Task<ChannelMetaDto?> GetChannelMetaAsync(string channelName) =>
+        Task.FromResult(ChannelMetaToReturn);
+
     public Task<(bool Success, string? Error, bool PasswordRequired)> EnsureChannelMembershipAsync(Guid userId, string channelName, string? password = null) =>
         Task.FromResult(MembershipResult);
 }