Add per-route top-10 leaderboard to activity detail

New /activities/{id}/route-leaderboard endpoint ranks the user's timed
efforts on the same route; frontend RouteLeaderboard card sits beside
Laps, showing this activity's time/rank/gap and the top 10 (current
effort highlighted green, also surfaced if outside the top 10).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/app/api/activities.py

@@ -229,6 +229,69 @@ async def get_lap_bests(
     return {str(lap_number): best for lap_number, best in rows}
 
 
+@router.get("/{activity_id}/route-leaderboard")
+async def get_route_leaderboard(
+    activity_id: int,
+    db: AsyncSession = Depends(get_db),
+    current_user: User = Depends(get_current_user),
+):
+    """Fastest-time leaderboard across all of this user's activities on the same
+    route. Returns this activity's rank/gap plus the top 10. Null if the activity
+    has no associated route (or no timed efforts to rank)."""
+    act = (await db.execute(
+        select(Activity).where(
+            Activity.id == activity_id,
+            Activity.user_id == current_user.id,
+        )
+    )).scalar_one_or_none()
+    if not act:
+        raise HTTPException(status_code=404, detail="Activity not found")
+    if not act.named_route_id:
+        return None
+
+    rows = (await db.execute(
+        select(
+            Activity.id, Activity.name, Activity.start_time,
+            Activity.duration_s, Activity.distance_m, Activity.avg_heart_rate,
+        )
+        .where(
+            Activity.named_route_id == act.named_route_id,
+            Activity.user_id == current_user.id,
+            Activity.duration_s.isnot(None),
+        )
+        .order_by(Activity.duration_s)
+    )).all()
+    if not rows:
+        return None
+
+    fastest_s = rows[0].duration_s
+    entries = []
+    current = None
+    for i, r in enumerate(rows):
+        entry = {
+            "rank": i + 1,
+            "activity_id": r.id,
+            "name": r.name,
+            "start_time": r.start_time,
+            "duration_s": r.duration_s,
+            "distance_m": r.distance_m,
+            "avg_heart_rate": r.avg_heart_rate,
+            "gap_s": r.duration_s - fastest_s,
+            "is_current": r.id == activity_id,
+        }
+        if entry["is_current"]:
+            current = entry
+        entries.append(entry)
+
+    return {
+        "route_id": act.named_route_id,
+        "total": len(entries),
+        "fastest_s": fastest_s,
+        "current": current,
+        "top": entries[:10],
+    }
+
+
 @router.patch("/{activity_id}/name")
 async def rename_activity(
     activity_id: int,