Add stats peak hours, RSS feed, channel health view, bulk video download

Stats:
- Peak watching hours chart (24-bar) from last_watched_at timestamps

RSS:
- GET /api/channels/rss — last 100 videos from followed channels as RSS 2.0
- RSS link in Following > Health tab

Channel health:
- New Health tab in Following groups channels into Active / Slow / Dormant / Dead
  based on days since last upload

Bulk video download:
- Select mode on Channel page (Videos tab) with checkboxes
- Sticky bottom bar shows count + Download button
- Queues a download for each selected video

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

frontend/src/api/index.js

@@ -65,6 +65,7 @@ export const addChannelToGroup = (groupId, channelId) => api.post(`/channels/gro
 export const removeChannelFromGroup = (groupId, channelId) => api.delete(`/channels/groups/${groupId}/channels/${channelId}`);
 export const bulkChannelAction = (channel_ids, action) => api.post("/channels/bulk-action", { channel_ids, action });
 export const getActiveTasks = () => api.get("/channels/tasks");
+export const getRssFeedUrl = () => `/api/channels/rss`;
 export const getRandomChannelVideo = (id, unwatchedOnly = true) =>
   api.get(`/channels/${id}/random`, { params: { unwatched_only: unwatchedOnly } });
 export const getChannelInProgress = (id) => api.get(`/channels/${id}/in-progress`);