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.Server/Config/UploadLimits.cs

@@ -0,0 +1,38 @@
+using EchoHub.Core.Constants;
+using EchoHub.Core.Models;
+
+namespace EchoHub.Server.Config;
+
+/// <summary>
+/// Admin-configurable upload limits, bound from the <c>Uploads</c> configuration section.
+/// Sizes are expressed in megabytes in configuration; the <c>*Bytes</c> accessors convert them
+/// for enforcement. Every default mirrors <see cref="HubConstants"/> so an absent or partial
+/// <c>Uploads</c> section preserves the historical built-in limits.
+/// </summary>
+public sealed class UploadLimits
+{
+    public int MaxFileSizeMB { get; init; } = HubConstants.MaxFileSizeBytes / (1024 * 1024);
+    public int MaxImageSizeMB { get; init; } = HubConstants.MaxImageSizeBytes / (1024 * 1024);
+    public int MaxAudioSizeMB { get; init; } = HubConstants.MaxAudioFileSizeBytes / (1024 * 1024);
+    public int MaxAvatarSizeMB { get; init; } = HubConstants.MaxAvatarSizeBytes / (1024 * 1024);
+    public int MaxAttachmentsPerMessage { get; init; } = HubConstants.MaxAttachmentsPerMessage;
+
+    public long MaxFileSizeBytes => (long)MaxFileSizeMB * 1024 * 1024;
+    public long MaxImageSizeBytes => (long)MaxImageSizeMB * 1024 * 1024;
+    public long MaxAudioSizeBytes => (long)MaxAudioSizeMB * 1024 * 1024;
+    public long MaxAvatarSizeBytes => (long)MaxAvatarSizeMB * 1024 * 1024;
+
+    /// <summary>Maximum accepted size for a single attachment of the given kind.</summary>
+    public long MaxForKind(AttachmentKind kind) => kind switch
+    {
+        AttachmentKind.Image => MaxImageSizeBytes,
+        AttachmentKind.Audio => MaxAudioSizeBytes,
+        _ => MaxFileSizeBytes,
+    };
+
+    /// <summary>
+    /// Absolute ceiling for one message request body (largest file × the attachment cap). Used to
+    /// size the request-body and multipart limits so a configured increase actually takes effect.
+    /// </summary>
+    public long MaxRequestBodyBytes => MaxFileSizeBytes * MaxAttachmentsPerMessage;
+}