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/UploadLimitsTests.cs

@@ -0,0 +1,66 @@
+using EchoHub.Core.Constants;
+using EchoHub.Core.Models;
+using EchoHub.Server.Config;
+using Microsoft.Extensions.Configuration;
+using Xunit;
+
+namespace EchoHub.Tests;
+
+public class UploadLimitsTests
+{
+    [Fact]
+    public void Defaults_MirrorHubConstants()
+    {
+        var limits = new UploadLimits();
+
+        Assert.Equal(HubConstants.MaxFileSizeBytes, limits.MaxFileSizeBytes);
+        Assert.Equal(HubConstants.MaxImageSizeBytes, limits.MaxImageSizeBytes);
+        Assert.Equal(HubConstants.MaxAudioFileSizeBytes, limits.MaxAudioSizeBytes);
+        Assert.Equal(HubConstants.MaxAvatarSizeBytes, limits.MaxAvatarSizeBytes);
+        Assert.Equal(HubConstants.MaxAttachmentsPerMessage, limits.MaxAttachmentsPerMessage);
+    }
+
+    [Fact]
+    public void MaxForKind_MapsEachAttachmentKind()
+    {
+        var limits = new UploadLimits
+        {
+            MaxImageSizeMB = 5,
+            MaxAudioSizeMB = 7,
+            MaxFileSizeMB = 11,
+        };
+
+        Assert.Equal(5L * 1024 * 1024, limits.MaxForKind(AttachmentKind.Image));
+        Assert.Equal(7L * 1024 * 1024, limits.MaxForKind(AttachmentKind.Audio));
+        Assert.Equal(11L * 1024 * 1024, limits.MaxForKind(AttachmentKind.File));
+    }
+
+    [Fact]
+    public void MaxRequestBodyBytes_IsFileSizeTimesAttachmentCap()
+    {
+        var limits = new UploadLimits { MaxFileSizeMB = 20, MaxAttachmentsPerMessage = 4 };
+
+        Assert.Equal(20L * 1024 * 1024 * 4, limits.MaxRequestBodyBytes);
+    }
+
+    [Fact]
+    public void BoundFromConfiguration_OverridesDefaults()
+    {
+        var config = new ConfigurationBuilder()
+            .AddInMemoryCollection(new Dictionary<string, string?>
+            {
+                ["Uploads:MaxFileSizeMB"] = "250",
+                ["Uploads:MaxImageSizeMB"] = "25",
+                ["Uploads:MaxAttachmentsPerMessage"] = "3",
+            })
+            .Build();
+
+        var limits = config.GetSection("Uploads").Get<UploadLimits>()!;
+
+        Assert.Equal(250L * 1024 * 1024, limits.MaxFileSizeBytes);
+        Assert.Equal(25L * 1024 * 1024, limits.MaxImageSizeBytes);
+        Assert.Equal(3, limits.MaxAttachmentsPerMessage);
+        // Unspecified values keep their HubConstants-derived defaults.
+        Assert.Equal(HubConstants.MaxAudioFileSizeBytes, limits.MaxAudioSizeBytes);
+    }
+}