feat: add server logs functionality with role-based access and system channel management

- Introduced a new migration to add the IsSystem column to the Channels table.
- Updated the DbContext model snapshot to reflect the new IsSystem property.
- Enhanced the ChannelService to manage system channels, including creation, visibility control, and protection against deletion.
- Implemented ServerLogsService to handle live server logging, including reading from log files and managing access based on user roles.
- Created ServerLogsSink to queue log events for streaming to the live log room.
- Developed ServerLogsStreamService to stream log events to clients in real-time.
- Added configuration options for server logs in appsettings.
- Implemented comprehensive unit tests for channel service system channel behavior and server logs functionality.

src/EchoHub.Server/Controllers/ChannelsController.cs

@@ -201,6 +201,9 @@ public class ChannelsController : ControllerBase
         if (channelDto is null)
             return NotFound(new ErrorResponse($"Channel '{channelName}' does not exist."));
 
+        if (channelDto.IsSystem)
+            return StatusCode(403, new ErrorResponse("This channel is read-only."));
+
         if (!Request.HasFormContentType)
             return BadRequest(new ErrorResponse("Expected multipart form data."));
 
@@ -344,6 +347,9 @@ public class ChannelsController : ControllerBase
         if (channelDto is null)
             return NotFound(new ErrorResponse($"Channel '{channelName}' does not exist."));
 
+        if (channelDto.IsSystem)
+            return StatusCode(403, new ErrorResponse("This channel is read-only."));
+
         if (channelDto.IsEncrypted)
             return BadRequest(new ErrorResponse(
                 "Sending images by URL is not available in end-to-end encrypted channels — download the image and /send the file instead."));