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>

backend/routers/channels.py

@@ -290,6 +290,57 @@ def sync_all_channels(
     return {"indexing": len(channels)}
 
 
+@router.get("/rss")
+def rss_feed(
+    db: Session = Depends(get_db),
+    current_user: User = Depends(get_current_user),
+):
+    from fastapi.responses import Response
+    rows = db.execute(
+        text("""
+            SELECT v.youtube_video_id, v.title, v.description, v.published_at,
+                   c.name AS channel_name, c.youtube_channel_id
+            FROM videos v
+            JOIN channels c ON v.channel_id = c.id
+            JOIN user_channels uc ON c.id = uc.channel_id AND uc.user_id = :uid AND uc.status = 'followed'
+            WHERE v.published_at IS NOT NULL
+            ORDER BY v.published_at DESC
+            LIMIT 100
+        """),
+        {"uid": current_user.id},
+    ).mappings().all()
+
+    def esc(s):
+        if not s:
+            return ""
+        return str(s).replace("&", "&amp;").replace("<", "&lt;").replace(">", "&gt;").replace('"', "&quot;")
+
+    items = []
+    for r in rows:
+        pub = r["published_at"]
+        pub_str = pub.strftime("%a, %d %b %Y %H:%M:%S +0000") if pub else ""
+        yt_id = r["youtube_video_id"]
+        items.append(f"""    <item>
+      <title>{esc(r['title'])}</title>
+      <link>https://www.youtube.com/watch?v={yt_id}</link>
+      <description>{esc(r['description'] or '')}</description>
+      <author>{esc(r['channel_name'])}</author>
+      <pubDate>{pub_str}</pubDate>
+      <guid>https://www.youtube.com/watch?v={yt_id}</guid>
+    </item>""")
+
+    xml = f"""<?xml version="1.0" encoding="UTF-8"?>
+<rss version="2.0">
+  <channel>
+    <title>YTContinue — Following</title>
+    <link>https://www.youtube.com/</link>
+    <description>Latest videos from your followed channels</description>
+{chr(10).join(items)}
+  </channel>
+</rss>"""
+    return Response(content=xml, media_type="application/rss+xml; charset=utf-8")
+
+
 @router.get("/tasks")
 def get_active_tasks(current_user: User = Depends(get_current_user)):
     with _tasks_lock: