Describe the bug

In fetcher/fetcher.go, when the DEBUG_IMAP environment variable is set, a file is opened for debug logging but never closed. Each IMAP connection leaks a file descriptor.

if path := os.Getenv("DEBUG_IMAP"); path != "" {
    if f, err := os.OpenFile(path, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0600); err == nil {
        options.DebugWriter = f
    }
}

The file f is never closed — there is no defer f.Close() and no tracking of the handle for later cleanup.

To reproduce

1. Set DEBUG_IMAP=/tmp/imap.log
2. Run matcha and let it make multiple IMAP connections (e.g. fetch emails, IDLE reconnects)
3. Observe file descriptor count growing via ls /proc/<pid>/fd | wc -l

Expected behavior

The debug log file handle should be properly closed when the IMAP connection is closed, or a single shared file handle should be reused across connections.

OS

All platforms