video_dl: skip download if video exceeds 500MB

[Copilot]

Previously, oversized videos were only detected after a full download completed. This adds a pre-flight size check using yt-dlp metadata to abort before wasting bandwidth and disk I/O.

Changes

• New constant MAX_DOWNLOAD_SIZE = 500 * 1024 * 1024 on the cog class
• Pre-download probe in _download_video(): calls extract_info(download=False) and checks filesize / filesize_approx against the limit before starting the actual download
• Fail-open on probe errors — if metadata fetch fails, a warning is logged and the download proceeds as before
• Explicit is not None guard rather than truthiness check, so a reported size of 0 doesn't bypass the limit

probe_info = ydl.extract_info(url, download=False)
file_size_estimate = probe_info.get('filesize') or probe_info.get('filesize_approx')
if file_size_estimate is not None and file_size_estimate > self.MAX_DOWNLOAD_SIZE:
    return False, None, f"Video is too large ({size_mb:.1f}MB). Maximum allowed size is {max_mb:.0f}MB."

The error propagates through the existing flow: silent in on_message, surfaced as a ❌ reply in the /download command.