feat(discovery): trending sort heuristic over world_visits with decay weighting

backend/app/routers/discovery.py

@@ -16,6 +16,8 @@ from app.database import get_db
 from app.dependencies import OptionalUser
 from app.models.world import World, WorldVisibility
 from app.models.world_visit import WorldVisit
+from app.services.trending import get_trending_subquery
+
 
 router = APIRouter()
 
@@ -208,11 +210,23 @@ async def list_discovery_worlds(
     total = (await db.execute(count_query)).scalar_one()
 
     # Sort
-    if sort == "most_played":
+    if sort == "trending":
+        trending_subq = get_trending_subquery()
+        base_query = (
+            base_query
+            .outerjoin(trending_subq, World.id == trending_subq.c.trending_world_id)
+            .order_by(
+                func.coalesce(trending_subq.c.trending_score, 0.0).desc(),
+                World.play_count.desc(),
+                World.created_at.desc(),
+            )
+        )
+    elif sort == "most_played":
         base_query = base_query.order_by(World.play_count.desc(), World.created_at.desc())
     else:  # default: newest
         base_query = base_query.order_by(World.created_at.desc())
 
+
     # Pagination & load owner
     query = (
         base_query

backend/app/services/trending.py

@@ -0,0 +1,34 @@
+"""Trending recommendation heuristic service.
+
+docs/planning/discovery.md (task 33706f401ad9).
+"""
+
+from sqlalchemy import case, func, select, text
+from app.models.world_visit import WorldVisit
+
+
+def get_trending_subquery(days: int = 7):
+    """Build a subquery that computes a decay-weighted trending score per world_id.
+
+    Decay weights:
+    - Visited within 24 hours: weight 3.0
+    - Visited within 3 days: weight 2.0
+    - Visited within 7 days: weight 1.0
+    """
+    trending_weight = case(
+        (WorldVisit.visited_at >= func.now() - text("interval '1 day'"), 3.0),
+        (WorldVisit.visited_at >= func.now() - text("interval '3 days'"), 2.0),
+        else_=1.0,
+    )
+
+    interval_str = f"interval '{days} days'"
+    subq = (
+        select(
+            WorldVisit.world_id.label("trending_world_id"),
+            func.coalesce(func.sum(trending_weight), 0.0).label("trending_score"),
+        )
+        .where(WorldVisit.visited_at >= func.now() - text(interval_str))
+        .group_by(WorldVisit.world_id)
+        .subquery("trending_scores")
+    )
+    return subq