feat: adopt Phase 3 gate and paper-trade exit policy

Production strategy change based on the July 2026 backtest: paper trades now default to a 30-trading-day hold with the initial stop (classic momentum hold-and-rerank), while target and trailing exits remain available in Admin. The exit policy API/UI now carries hold_days and close_reason can be 'time'.

The activation confidence floor default is now 0/off because the gate ablation showed it added no per-trade edge while filtering out usable setups. Migration 015 clears stored activation_min_confidence and paper_exit_mode so the new defaults take effect; this intentionally resets Track Record comparability from this deploy.

Verification: 451 backend tests pass, ruff check app/ clean, frontend npm run build clean.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/lib/types.ts

@@ -211,14 +211,15 @@ export interface PaperTrade {
   benchmark_return_pct: number | null;
   alpha_pct: number | null;
   alpha_usd: number | null;
-  close_reason: 'trailing' | 'stop' | 'target' | 'manual' | null;
+  close_reason: 'time' | 'trailing' | 'stop' | 'target' | 'manual' | null;
   trailing_stop: number | null;
   trailing_distance_pct: number | null;
 }
 
 export interface ExitPolicy {
-  mode: 'trailing' | 'target';
+  mode: 'time' | 'trailing' | 'target';
   trailing_pct: number;
+  hold_days: number;
 }
 
 export interface BacktestBucket {