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>

frontend/src/api/index.js

@@ -155,3 +155,10 @@ export const deleteCollection = (id) => api.delete(`/collections/${id}`);
 export const getCollectionVideos = (id) => api.get(`/collections/${id}/videos`);
 export const addToCollection = (collectionId, videoId) => api.post(`/collections/${collectionId}/videos`, { video_id: videoId });
 export const removeFromCollection = (collectionId, videoId) => api.delete(`/collections/${collectionId}/videos/${videoId}`);
+
+// Playlists
+export const getChannelPlaylists = (channelId) => api.get(`/playlists/channel/${channelId}`);
+export const fetchChannelPlaylists = (channelId) => api.post(`/playlists/channel/${channelId}/fetch`);
+export const getPlaylistVideos = (playlistId, offset = 0, limit = 60) =>
+  api.get(`/playlists/${playlistId}/videos`, { params: { offset, limit } });
+export const indexPlaylist = (playlistId) => api.post(`/playlists/${playlistId}/index`);