Add older content exploration: channel page + home feed Rediscover mode

Channel page:
- "Explore older videos" button fetches 100 videos at a time further back
  in the channel history using yt-dlp --playlist-start/--playlist-end
- "Fetch entire history" still available for full crawl
- Backend: /channels/{id}/explore?page=N endpoint + playlist offset support
  in fetch_channel_metadata(start_video=N)

Home feed:
- New "Rediscover" mode: older unwatched videos (90+ days old) from
  followed channels, randomly sampled then re-ranked by tag affinity

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

backend/routers/videos.py

@@ -208,6 +208,58 @@ def home_feed(
             for r in rows
         ]
 
+    if mode == "rediscover":
+        # Older unwatched videos from followed channels, ranked by tag affinity
+        affinity_rows = db.execute(
+            text("SELECT tag, score FROM user_tag_affinity WHERE user_id = :user_id AND score > 0"),
+            {"user_id": current_user.id},
+        ).mappings().all()
+        affinity = {r["tag"]: r["score"] for r in affinity_rows}
+
+        rows = db.execute(
+            text(f"""
+                SELECT v.id, v.youtube_video_id, v.title, v.description, v.thumbnail_url,
+                       v.duration_seconds, v.published_at, v.tags, v.category,
+                       c.id AS channel_id, c.name AS channel_name,
+                       c.youtube_channel_id AS channel_youtube_id,
+                       c.thumbnail_url AS channel_thumbnail_url,
+                       COALESCE(uv.watched, 0)                AS watched,
+                       COALESCE(uv.watch_progress_seconds, 0) AS watch_progress_seconds,
+                       COALESCE(uv.downloaded, 0)             AS is_downloaded,
+                       COALESCE(uv.queued, 0)                 AS queued,
+                       NULL                                   AS file_path
+                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 = :user_id AND uc.status = 'followed'
+                LEFT JOIN user_videos uv ON v.id = uv.video_id AND uv.user_id = :user_id
+                WHERE COALESCE(uv.watched, 0) = 0
+                  AND v.published_at < datetime('now', '-90 days')
+                  AND (uc.muted_until IS NULL OR datetime(uc.muted_until) < datetime('now'))
+                  {duration_clause}
+                ORDER BY RANDOM()
+                LIMIT :limit OFFSET :offset
+            """),
+            {"user_id": current_user.id, "limit": min(limit * 4, 200), "offset": offset},
+        ).mappings().all()
+
+        if affinity:
+            import json as _json
+            def _affinity_score(row):
+                try:
+                    tags = _json.loads(row["tags"] or "[]")
+                    return sum(affinity.get(t.lower().strip(), 0) for t in tags if isinstance(t, str))
+                except Exception:
+                    return 0
+            rows = sorted(rows, key=_affinity_score, reverse=True)
+
+        rows = rows[:limit]
+        return [
+            VideoDetail(**{k: v for k, v in dict(r).items() if k not in ("watched",)},
+                        is_watched=bool(r["watched"]))
+            for r in rows
+        ]
+
     if mode == "inbox":
         rows = db.execute(
             text(f"""