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/CommandHandlerTests.cs

@@ -472,4 +472,22 @@ public class CommandHandlerTests
         Assert.True(result.Handled);
         Assert.True(quitCalled);
     }
+
+    // ── /meta ─────────────────────────────────────────────────────────
+
+    [Theory]
+    [InlineData("/meta")]
+    [InlineData("/info")]
+    public async Task HandleAsync_Meta_RaisesRoomInfo(string input)
+    {
+        var handler = CreateHandler();
+        var raised = false;
+        handler.OnRoomInfo += () => { raised = true; return Task.CompletedTask; };
+
+        var result = await handler.HandleAsync(input);
+
+        Assert.True(result.Handled);
+        Assert.False(result.IsError);
+        Assert.True(raised);
+    }
 }