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.Core/Contracts/IChannelService.cs

@@ -24,6 +24,11 @@ public interface IChannelService
 
     // Membership
     Task<(bool Success, string? Error, bool PasswordRequired)> EnsureChannelMembershipAsync(Guid userId, string channelName, string? password = null);
+
+    // System channels (server-managed, e.g. the live server-log room). Creates the channel
+    // if missing and reclaims a same-named regular channel so server content never lands in
+    // a user-owned room.
+    Task<ChannelDto> EnsureSystemChannelAsync(string channelName, string? topic = null);
 }
 
 public record ChannelListItem(string Name, string? Topic, int OnlineCount, bool IsPublic = true, bool IsProtected = false);