momentum gate: long-only + wire the percentile onto live setups

Part 1 — long-only. The momentum edge is long top-momentum; the gate was
qualifying shorts on high-momentum names (fighting the trend), which showed as
the -0.13R Short(qual.) drag. While the gate is active, shorts no longer qualify
(backend qualification, backtest _momentum_qualifies, and the frontend mirror).

Part 2 — production wiring. Live setups now carry a real momentum rank, so the
dashboard, the Track Record's qualified stats, and outcome evaluation all gate on
the same value instead of deferring to floors:
- new momentum_service.compute_momentum_percentiles: 12-1 momentum per ticker,
  ranked across the universe into a {symbol: percentile} map.
- the daily R:R scan ranks the universe up front and stores each setup's
  percentile (new trade_setups.momentum_percentile column, migration 010).
- enhance_trade_setup mutates the same row, so the percentile is preserved;
  _trade_setup_to_dict + TradeSetupResponse expose it to the API.

Until a fresh scan runs, pre-existing setups have a null percentile and the gate
falls back to floors for them (longs) / excludes them (shorts) — they fill in on
the next scan. 341 backend tests pass; frontend build clean.

Needs the alembic upgrade (migration 010) on deploy.

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

frontend/src/lib/qualification.ts

@@ -33,11 +33,14 @@ export function qualifiesSetup(setup: TradeSetup, config: ActivationConfig): boo
     return false;
   }
   if ((setup.confidence_score ?? 0) < config.min_confidence) return false;
-  // Cross-sectional momentum is the core selection — only enforced when a
-  // percentile is attached and a threshold is set; otherwise defer to the floors.
-  if (config.min_momentum_percentile > 0 && setup.momentum_percentile != null
-      && setup.momentum_percentile < config.min_momentum_percentile) {
-    return false;
+  // Cross-sectional momentum is the core selection (long-only). While the gate is
+  // active, shorts never qualify; the percentile floor is enforced only when a
+  // percentile is attached, otherwise defer to the floors.
+  if (config.min_momentum_percentile > 0) {
+    if (setup.direction === 'short') return false;
+    if (setup.momentum_percentile != null && setup.momentum_percentile < config.min_momentum_percentile) {
+      return false;
+    }
   }
   if (config.require_high_conviction && !HIGH_CONVICTION_ACTIONS.has(setup.recommended_action ?? '')) {
     return false;