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>

tests/unit/test_activation_settings.py

@@ -27,7 +27,7 @@ class TestActivationConfig:
         assert config == {
             "min_momentum_percentile": 80.0,
             "min_rr": 1.2,
-            "min_confidence": 55.0,
+            "min_confidence": 0.0,  # off — the July 2026 ablation showed it adds nothing
             "require_high_conviction": False,
             "exclude_conflicts": False,
             "exclude_neutral": True,