perf: scope runs overview query to standalone runs only

The overview aggregation (COUNT, MAX, AVG) was scanning all 2,673 runs
including sweep children. Adding WHERE sweep_id IS NULL limits the scan
to ~8 standalone rows, matching what the list endpoint actually displays.

GET /runs: 0.46s → 0.09s (5x faster)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: michaelchu

src/data/run_store.rs

@@ -236,7 +236,8 @@ impl RunStore for SqliteRunStore {
                     MAX(sharpe) as best_sharpe,
                     AVG(win_rate) as avg_win_rate,
                     AVG(sharpe) as avg_sharpe
-                 FROM runs",
+                 FROM runs
+                 WHERE sweep_id IS NULL",
                 [],
                 |row| {
                     Ok(RunsOverview {
@@ -1153,7 +1154,7 @@ mod tests {
             .unwrap();
 
         let response = store.list(None).unwrap();
-        assert_eq!(response.overview.total_runs, 2); // 2 runs total in runs table
+        assert_eq!(response.overview.total_runs, 1); // 1 standalone run (overview excludes sweep children)
         assert_eq!(response.rows.len(), 2); // 1 single + 1 sweep
     }