Fix @handle URL resolution using --flat-playlist

- Updated extract_channel_id() to use --flat-playlist instead of --playlist-items 0
- Append /videos to URLs to ensure playlist metadata is available
- Extract playlist_channel_id and playlist_channel from flat-playlist output
- Increased timeout to 15 seconds for network fetch
- All URL formats now working: direct IDs, /channel/ URLs, @handle URLs

Tested and verified in Docker container.

app.py

@@ -69,19 +69,28 @@ def extract_channel_id(url_or_id):
         # Format: youtube.com/@handle or youtube.com/c/name or youtube.com/user/name
         # These require fetching the page to get the actual channel ID
         if parsed.netloc in ['youtube.com', 'www.youtube.com', 'm.youtube.com']:
-            # Use yt-dlp to extract channel ID from URL
+            # Use yt-dlp with --flat-playlist to extract channel ID without downloading
+            # Append /videos if not already present to ensure we get playlist metadata
+            fetch_url = url_or_id
+            if not fetch_url.endswith('/videos'):
+                fetch_url = fetch_url.rstrip('/') + '/videos'
+
             try:
                 result = subprocess.run(
-                    ['yt-dlp', '--dump-json', '--playlist-items', '0', url_or_id],
+                    ['yt-dlp', '--flat-playlist', '--dump-json', '--playlist-items', '1', fetch_url],
                     capture_output=True,
                     text=True,
-                    timeout=10
+                    timeout=15
                 )
 
                 if result.returncode == 0 and result.stdout:
-                    data = json.loads(result.stdout.split('\n')[0])
-                    channel_id = data.get('channel_id')
-                    channel_name = data.get('channel')
+                    # Get first line of JSON output
+                    lines = [l for l in result.stdout.split('\n') if l.strip()]
+                    if lines:
+                        data = json.loads(lines[0])
+                        # flat-playlist returns playlist_channel_id instead of channel_id
+                        channel_id = data.get('playlist_channel_id') or data.get('channel_id')
+                        channel_name = data.get('playlist_channel') or data.get('channel')
 
                         if channel_id:
                             return channel_id, channel_name