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/appsettings.example.json

@@ -20,6 +20,13 @@
     "CleanupIntervalHours": 1,
     "RetentionDays": 30
   },
+  "Uploads": {
+    "MaxFileSizeMB": 100,
+    "MaxImageSizeMB": 10,
+    "MaxAudioSizeMB": 10,
+    "MaxAvatarSizeMB": 2,
+    "MaxAttachmentsPerMessage": 10
+  },
   "Encryption": {
     "Key": "",
     "EncryptDatabase": false