Fix discovery scoring: cap trending, prevent score inflation, add freshness

- Cap trending base_score at 18.0 (was unbounded — a viral channel could
  score 240+ vs search's 15, making everything else invisible)
- Cap all discovery scores at 50.0 globally so no single signal dominates
- Fix score accumulation: cap accumulated total at 50.0 (was unbounded
  across repeated runs, cementing high-score channels in top positions forever)
- Expire unseen queue entries older than 14 days at start of each run
- Add ±8 score perturbation to discovery list endpoint (was pure score DESC,
  identical every visit until dismissed)
- Add score perturbation to discovery_videos ORDER BY too
- Fix SQL injection in update_category_clusters (category strings were
  interpolated directly into query; now use parameterized queries per category)
- Raise category signal score from 3.0 → 5.0 to compensate for trending cap

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

backend/routers/discovery.py

@@ -1,4 +1,5 @@
 import json
+import random
 from typing import Optional
 
 from fastapi import APIRouter, BackgroundTasks, Depends, HTTPException
@@ -81,6 +82,9 @@ def list_discovery(
                 pass
         rows = [r for r in rows if neg_hit.get(r["channel_id"], 0) < 3]
 
+    # Add score perturbation so the list doesn't look identical every visit.
+    # ±8 jitter keeps relative ranking meaningful while surfacing different channels.
+    rows = sorted(rows, key=lambda r: r["score"] + random.uniform(-8, 8), reverse=True)
     rows = rows[:limit]
     items = []
     for row in rows:
@@ -210,7 +214,7 @@ def discovery_videos(
                   )
             )
             WHERE rn <= 2
-            ORDER BY score DESC, rn ASC, RANDOM()
+            ORDER BY (score + (RANDOM() * 10 - 5)) DESC, rn ASC
             LIMIT :limit OFFSET :offset
         """),
         {"user_id": current_user.id, "limit": limit, "offset": offset},

backend/services/discovery.py

@@ -77,14 +77,18 @@ def _upsert_channel(db: Session, channel_data: dict) -> Channel | None:
     return channel
 
 
+_MAX_DISCOVERY_SCORE = 50.0
+
+
 def _add_to_discovery(
     db: Session, user_id: int, channel_id: int, score: float, source: str,
     preview_json: str | None = None,
 ):
+    score = min(score, _MAX_DISCOVERY_SCORE)
     existing = db.query(DiscoveryQueue).filter_by(user_id=user_id, channel_id=channel_id).first()
     if existing:
-        # Accumulate scores across sources but cap to prevent one dominant signal
-        existing.score = existing.score + score * 0.5
+        # Accumulate across sources but cap so no single signal dominates forever
+        existing.score = min(existing.score + score * 0.5, _MAX_DISCOVERY_SCORE)
         if preview_json and not existing.preview_json:
             existing.preview_json = preview_json
         return
@@ -322,22 +326,26 @@ def update_category_clusters(db: Session, user_id: int):
     if not top_categories:
         return
 
-    placeholders = ",".join(f"'{c}'" for c in top_categories)
-    candidate_rows = db.execute(
-        text(f"""
-            SELECT DISTINCT v.channel_id
-            FROM videos v
-            WHERE v.category IN ({placeholders})
-              AND v.channel_id NOT IN (
-                  SELECT channel_id FROM user_channels WHERE user_id = :user_id
-              )
-            LIMIT 100
-        """),
-        {"user_id": user_id},
-    ).mappings().all()
+    # Use JSON_EACH / parameterized IN via repeated queries to avoid SQL injection
+    candidate_channel_ids: set[int] = set()
+    for cat in top_categories:
+        cat_rows = db.execute(
+            text("""
+                SELECT DISTINCT v.channel_id
+                FROM videos v
+                WHERE v.category = :cat
+                  AND v.channel_id NOT IN (
+                      SELECT channel_id FROM user_channels WHERE user_id = :user_id
+                  )
+                LIMIT 50
+            """),
+            {"cat": cat, "user_id": user_id},
+        ).mappings().all()
+        for row in cat_rows:
+            candidate_channel_ids.add(row["channel_id"])
 
-    for row in candidate_rows:
-        _add_to_discovery(db, user_id, row["channel_id"], score=3.0, source="category")
+    for channel_id in candidate_channel_ids:
+        _add_to_discovery(db, user_id, channel_id, score=5.0, source="category")
     db.commit()
 
 
@@ -548,7 +556,9 @@ def update_trending_signal(db: Session, user_id: int, regions: list[str]):
         if uc and uc.status in ("followed", "dismissed"):
             continue
 
-        base_score = float(info["count"]) * 4.0 * len(info["regions"])
+        # Cap base_score so a viral trending channel can't dominate the whole queue.
+        # count × 4.0 × regions can reach 300+ without this cap.
+        base_score = min(float(info["count"]) * 4.0 * len(info["regions"]), 18.0)
 
         # Tag relevance: positive for liked content, negative for dismissed/disliked.
         # tag_profile comes from user_tag_affinity which tracks both signals.
@@ -561,7 +571,7 @@ def update_trending_signal(db: Session, user_id: int, regions: list[str]):
             for tags_json in tag_rows:
                 tag_boost += _tag_relevance_score(tag_profile, tags_json)
 
-        final_score = base_score + tag_boost
+        final_score = min(base_score + tag_boost, 25.0)
         if final_score <= 0:
             continue
 
@@ -646,6 +656,19 @@ def update_graph_signal(db: Session, user_id: int):
 def run_full_discovery(db: Session, user_id: int, regions: list[str] | None = None):
     if regions is None:
         regions = ["US", "SE"]
+
+    # Expire unseen entries older than 14 days so stale high-score channels
+    # don't block fresh results forever.
+    db.execute(
+        text("""
+            DELETE FROM discovery_queue
+            WHERE user_id = :user_id AND seen = 0
+              AND created_at <= datetime('now', '-14 days')
+        """),
+        {"user_id": user_id},
+    )
+    db.commit()
+
     crawl_by_search(db, user_id)
     update_community_signal(db, user_id)
     update_category_clusters(db, user_id)