Add playlists support and fix explore older videos

- New playlists router: fetch channel playlists from YouTube, index
  playlist videos, browse by playlist with pagination
- Playlist model gets video_ids column to store ordered video list
- Register playlists router in main.py with DB migration
- Add Playlists tab to Channel page: grid of playlist cards, click to
  browse videos, index/re-index per playlist
- Fix explore older videos skipping all entries without published_at;
  flat-playlist entries for older videos rarely include timestamp data

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/services/ytdlp.py

@@ -353,6 +353,83 @@ def fetch_channel_metadata(channel_id: str, max_videos: int = 30, start_video: i
     return {"channel": channel_info, "videos": videos}
 
 
+def fetch_channel_playlists(channel_id: str, max_results: int = 100) -> list[dict]:
+    """Fetch the playlists listed on a channel's /playlists tab."""
+    if channel_id.startswith("@"):
+        url = f"https://www.youtube.com/{channel_id}/playlists"
+    else:
+        url = f"https://www.youtube.com/channel/{channel_id}/playlists"
+    stdout, _, code = _run([
+        "yt-dlp", url,
+        "--dump-json", "--flat-playlist",
+        "--playlist-end", str(max_results),
+        "--quiet",
+        *_cookie_args(),
+    ], timeout=60)
+
+    playlists = []
+    for line in stdout.splitlines():
+        line = line.strip()
+        if not line:
+            continue
+        try:
+            info = json.loads(line)
+            pl_id = info.get("id") or info.get("playlist_id")
+            title = info.get("title") or info.get("playlist_title") or ""
+            if not pl_id or not title or pl_id == channel_id:
+                continue
+            playlists.append({
+                "youtube_playlist_id": pl_id,
+                "title": title,
+                "description": info.get("description"),
+                "thumbnail_url": _stable_thumbnail(info.get("id")) if info.get("_type") == "url" else None,
+                "video_count": info.get("playlist_count") or info.get("n_entries") or 0,
+            })
+        except json.JSONDecodeError:
+            continue
+    return playlists
+
+
+def fetch_playlist_videos(playlist_id: str, max_videos: int = 200) -> list[dict]:
+    """Fetch videos from a YouTube playlist by playlist ID."""
+    url = f"https://www.youtube.com/playlist?list={playlist_id}"
+    args = [
+        "yt-dlp", url,
+        "--dump-json", "--flat-playlist",
+        "--quiet",
+        *_cookie_args(),
+    ]
+    if max_videos > 0:
+        args += ["--playlist-end", str(max_videos)]
+    stdout, _, code = _run(args, timeout=120)
+
+    videos = []
+    for line in stdout.splitlines():
+        line = line.strip()
+        if not line:
+            continue
+        try:
+            info = json.loads(line)
+            vid_id = info.get("id")
+            if not vid_id:
+                continue
+            videos.append({
+                "youtube_video_id": vid_id,
+                "title": info.get("title", ""),
+                "thumbnail_url": _stable_thumbnail(vid_id),
+                "duration_seconds": info.get("duration"),
+                "published_at": _parse_published(info),
+                "view_count": info.get("view_count"),
+                "channel": {
+                    "youtube_channel_id": info.get("channel_id"),
+                    "name": info.get("channel") or info.get("uploader") or "",
+                },
+            })
+        except json.JSONDecodeError:
+            continue
+    return videos
+
+
 def fetch_featured_channels(channel_id: str) -> list[str]:
     """Fetch channel IDs from the /channels tab of a YouTube channel.