Fix affinity scoring, add completion signal, seamless local player switch

Ranked feed — affinity was broken:
- Was looking up user_tag_affinity by v.category (e.g. "Science & Technology")
  but affinity is stored using fine-grained video tags ("linux", "rust", etc.)
- Now uses SUM across all matching affinities: category OR any tag found in the
  video's tags JSON via instr() — up to 5 matches to prevent runaway scores

Ranked feed — completion rate now influences channel scoring:
- Added avg_completion_pct to channel_stats CTE (AVG of completion_percent)
- Channels where you finish videos score higher; channels you bail on score lower
- Defaults to 50% (neutral) for channels with no tracked completions

Progress endpoint — backend auto-watched safety net:
- If completion_percent reaches ≥90% on a video >60s, mark watched automatically
- Catches cases where browser closes before the 10s debounce fires
- Guards against double-calling _update_affinity with not prev_watched check

VideoPlayer — seamless local file switch:
- Removed switchedToLocal state which caused a race condition: video loaded with
  local_file_url already set but flag was still false, requiring a page refresh
- local_file_url from the backend is the single source of truth (backend gates
  it with os.path.exists so it only appears when the file is actually on disk)
- Show spinner while video metadata loads, then immediately show local player
  if file exists — no YouTube flash for already-downloaded videos
- After download completes, single refetchVideo() picks up the new URL and
  React re-renders directly into local player

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

backend/routers/videos.py

@@ -253,7 +253,8 @@ def home_feed(
                     v.channel_id,
                     COUNT(CASE WHEN uv.watched = 1 THEN 1 END) AS watched_count,
                     COUNT(CASE WHEN uv.liked   = 1 THEN 1 END) AS liked_count,
-                    SUM(CASE WHEN uv.rating IS NOT NULL THEN uv.rating ELSE 0 END) AS rating_sum
+                    SUM(CASE WHEN uv.rating IS NOT NULL THEN uv.rating ELSE 0 END) AS rating_sum,
+                    AVG(CASE WHEN uv.completion_percent IS NOT NULL THEN uv.completion_percent END) AS avg_completion_pct
                 FROM videos v
                 LEFT JOIN user_videos uv ON v.id = uv.video_id AND uv.user_id = :user_id
                 GROUP BY v.channel_id
@@ -271,16 +272,19 @@ def home_feed(
                     COALESCE(uv.queued, 0)                 AS queued,
                     uv.rating                              AS rating,
                     NULL                                   AS file_path,
-                    (SQRT(CAST(COALESCE(cs.watched_count, 0) AS REAL)) * 6.0
-                        + COALESCE(cs.liked_count, 0) * 12.0
-                        + COALESCE(cs.rating_sum, 0) * 8.0) * :w_channel
+                    (SQRT(CAST(COALESCE(cs.watched_count, 0) AS REAL)) * 5.0
+                        + COALESCE(cs.liked_count, 0) * 10.0
+                        + COALESCE(cs.rating_sum, 0) * 8.0
+                        + COALESCE(cs.avg_completion_pct, 50.0) * 0.08) * :w_channel
                         + MAX(COALESCE(julianday(v.published_at) - julianday('now'), -90), -365) * :w_recency
                         + COALESCE((
-                            SELECT uta.score FROM user_tag_affinity uta
+                            SELECT COALESCE(SUM(uta.score), 0)
+                            FROM user_tag_affinity uta
                             WHERE uta.user_id = :user_id
-                              AND uta.tag = LOWER(COALESCE(v.category, ''))
-                            LIMIT 1
-                          ), 0) * 3.0 * :w_affinity
+                              AND (uta.tag = LOWER(COALESCE(v.category, ''))
+                                   OR instr(LOWER(COALESCE(v.tags, '')), '"' || uta.tag || '"') > 0)
+                            LIMIT 5
+                          ), 0) * :w_affinity
                         AS score,
                     ROW_NUMBER() OVER (
                         PARTITION BY v.channel_id
@@ -898,6 +902,19 @@ def update_progress(
         if pct < 0.20:
             _update_affinity(db, current_user.id, video, -0.5)
 
+    # Backend safety net: auto-mark watched at ≥90% completion even if the frontend
+    # didn't send watched=True (e.g. browser closed before debounce fired)
+    if (not prev_watched and not uv.watched
+            and uv.completion_percent is not None and uv.completion_percent >= 90
+            and video.duration_seconds and video.duration_seconds > 60):
+        uv.watched = True
+        _update_affinity(db, current_user.id, video, +2.0)
+        dl = db.query(Download).filter_by(
+            user_id=current_user.id, video_id=video_id, status="complete"
+        ).filter(Download.pending_delete_at.is_(None)).first()
+        if dl:
+            dl.pending_delete_at = datetime.utcnow() + timedelta(days=7)
+
     db.commit()
     return {"ok": True}