backtest: add min target-probability sweep

Re-applies the activation gate at several min_target_probability thresholds
(60→30, other conditions fixed) over the already-replayed candidates, so the
trade-off between how many setups qualify and their expectancy is visible in one
table — the cheap "optimize" half of Phase 2. Candidates now carry meets_core +
best_prob so the sweep needs no re-replay. New sweep table in BacktestPanel with
the current threshold starred.

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

frontend/src/lib/types.ts

@@ -196,6 +196,10 @@ export interface BacktestCalibrationRow {
   realized_hit_rate: number;
 }
 
+export interface BacktestSweepRow extends BacktestBucket {
+  min_target_probability: number;
+}
+
 export interface BacktestReport {
   generated_at: string;
   tickers: number;
@@ -205,6 +209,8 @@ export interface BacktestReport {
   overall_qualified: BacktestBucket;
   overall_all: BacktestBucket;
   by_direction: Record<string, BacktestBucket>;
+  min_target_probability: number;
+  sweep: BacktestSweepRow[];
   calibration: BacktestCalibrationRow[];
   note: string;
 }