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/Config/ServerLogsOptions.cs

@@ -0,0 +1,43 @@
+using EchoHub.Core.Models;
+using Serilog.Events;
+
+namespace EchoHub.Server.Config;
+
+/// <summary>
+/// Live server-log room settings, bound from the "ServerLogs" config section (env override:
+/// <c>ServerLogs__Enabled</c> etc.). When enabled, a read-only system channel is auto-created
+/// and log events are streamed to it live — log lines are never stored as messages in the
+/// database; the rolling Serilog log files remain the only persistence.
+/// </summary>
+public sealed class ServerLogsOptions
+{
+    /// <summary>Master switch for the live log room.</summary>
+    public bool Enabled { get; set; } = true;
+
+    /// <summary>
+    /// Name of the auto-created system channel. Must satisfy the normal channel-name rules;
+    /// the name is reserved — users cannot create a channel with it.
+    /// </summary>
+    public string RoomName { get; set; } = "server-logs";
+
+    /// <summary>Minimum server role allowed to see and join the log room (Member/Mod/Admin/Owner).</summary>
+    public ServerRole MinRole { get; set; } = ServerRole.Mod;
+
+    /// <summary>
+    /// Minimum level of log events streamed to the room (Verbose/Debug/Information/Warning/
+    /// Error/Fatal). Only affects the room — file and console sinks keep their own levels.
+    /// </summary>
+    public LogEventLevel MinLevel { get; set; } = LogEventLevel.Information;
+
+    /// <summary>How many recent log entries are replayed from the log file when someone opens the room.</summary>
+    public int BacklogLines { get; set; } = 100;
+
+    /// <summary>Directory holding the rolling log files. Must match the Serilog file sink's path.</summary>
+    public string LogDirectory { get; set; } = "logs";
+
+    /// <summary>Filename glob for the rolling log files inside <see cref="LogDirectory"/>.</summary>
+    public string LogFilePattern { get; set; } = "echohub-server-*.log";
+
+    /// <summary>Channel names are stored lowercased; compare against this form.</summary>
+    public string NormalizedRoomName => RoomName.ToLowerInvariant().Trim();
+}