feat: log backfill endpoint for desktop log panel

- LogStreamService: ring buffer (500 entries) + getRecentLogs() method
- SupervisorController: GET /api/supervisor/logs/recent returns buffered
  log entries so the desktop log panel shows history on tab open, not
  just live stream

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/logging/log-stream.service.ts

@@ -15,14 +15,23 @@ export type LogEntry = {
 export class LogStreamService extends ConsoleLogger {
     static readonly instance = new LogStreamService();
     readonly logSubject = new Subject<LogEntry>();
+    private readonly buffer: LogEntry[] = [];
+    private static readonly MAX_BUFFER = 500;
+
+    getRecentLogs(limit = 200): LogEntry[] {
+        return this.buffer.slice(-limit);
+    }
 
     private emit(level: LogEntry['level'], message: unknown, context?: string) {
-        this.logSubject.next({
+        const entry: LogEntry = {
             timestamp: new Date().toISOString(),
             level,
             context: context ?? this.context ?? '',
             message: typeof message === 'string' ? message : JSON.stringify(message),
-        });
+        };
+        this.buffer.push(entry);
+        if (this.buffer.length > LogStreamService.MAX_BUFFER) this.buffer.shift();
+        this.logSubject.next(entry);
     }
 
     log(message: unknown, context?: string) {