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_paper_trade_service.py

@@ -205,9 +205,12 @@ class TestTrailingClose:
 
 
 async def test_exit_policy_defaults_and_round_trip(session):
-    assert await svc.get_exit_policy(session) == {"mode": "trailing", "trailing_pct": 12.0}
-    updated = await svc.set_exit_policy(session, mode="target", trailing_pct=15.0)
-    assert updated == {"mode": "target", "trailing_pct": 15.0}
+    # Default: the backtest-validated hold-to-horizon exit.
+    assert await svc.get_exit_policy(session) == {
+        "mode": "time", "trailing_pct": 12.0, "hold_days": 30,
+    }
+    updated = await svc.set_exit_policy(session, mode="target", trailing_pct=15.0, hold_days=21)
+    assert updated == {"mode": "target", "trailing_pct": 15.0, "hold_days": 21}
     assert (await svc.get_exit_policy(session))["mode"] == "target"
 
 
@@ -216,10 +219,64 @@ async def test_exit_policy_rejects_bad_input(session):
         await svc.set_exit_policy(session, mode="bogus")
     with pytest.raises(ValidationError):
         await svc.set_exit_policy(session, trailing_pct=200.0)
+    with pytest.raises(ValidationError):
+        await svc.set_exit_policy(session, hold_days=1)
+
+
+def _r(d: date, open_: float, hi: float, lo: float, close: float) -> tuple:
+    return (d, open_, hi, lo, close)
+
+
+class TestTimeClose:
+    def test_closes_at_hold_days_close(self):
+        rows = [
+            _r(date(2026, 1, 2), 101, 103, 100, 102),
+            _r(date(2026, 1, 3), 102, 104, 101, 103),
+            _r(date(2026, 1, 4), 103, 106, 102, 105),
+        ]
+        assert svc._time_close("long", 95.0, 3, rows) == (105.0, date(2026, 1, 4), "time")
+
+    def test_stop_before_horizon(self):
+        rows = [_r(date(2026, 1, 2), 100, 101, 94, 96)]
+        assert svc._time_close("long", 95.0, 30, rows) == (95.0, date(2026, 1, 2), "stop")
+
+    def test_gap_through_stop_fills_at_open(self):
+        rows = [_r(date(2026, 1, 2), 92, 93, 90, 91)]
+        assert svc._time_close("long", 95.0, 30, rows) == (92.0, date(2026, 1, 2), "stop")
+
+    def test_none_before_horizon(self):
+        rows = [_r(date(2026, 1, 2), 101, 103, 100, 102)]
+        assert svc._time_close("long", 95.0, 5, rows) is None
+
+
+async def test_resolve_time_mode_closes_at_horizon(session):
+    await svc.set_exit_policy(session, mode="time", hold_days=2)
+    tid = await _seed(session, "AAA", close=100.0)
+    trade = await svc.create_trade(session, 1, symbol="AAA", direction="long",
+                                   entry_price=100.0, shares=10, stop_loss=95.0, target=200.0)
+    await _add_bars(session, tid, [(103, 101), (105, 102)], start=date.today())
+    assert await svc.resolve_open_trades(session) == 1
+    await session.refresh(trade)
+    assert trade.status == "closed"
+    assert trade.close_reason == "time"
+    assert trade.close_price == pytest.approx((105 + 102) / 2)  # day-2 close (= bar mid)
+
+
+async def test_resolve_time_mode_stop_still_governs(session):
+    await svc.set_exit_policy(session, mode="time", hold_days=30)
+    tid = await _seed(session, "AAA", close=100.0)
+    trade = await svc.create_trade(session, 1, symbol="AAA", direction="long",
+                                   entry_price=100.0, shares=10, stop_loss=95.0, target=200.0)
+    await _add_bars(session, tid, [(101, 94)], start=date.today())  # low pierces the stop
+    assert await svc.resolve_open_trades(session) == 1
+    await session.refresh(trade)
+    assert trade.close_reason == "stop"
+    assert trade.close_price == pytest.approx(95.0)
 
 
 async def test_resolve_trailing_closes_with_reason(session):
-    tid = await _seed(session, "AAA", close=100.0)  # default policy: trailing 12%
+    await svc.set_exit_policy(session, mode="trailing", trailing_pct=12.0)
+    tid = await _seed(session, "AAA", close=100.0)
     await _add_open_trade(session, tid, "long", entry=100.0, shares=10, days_ago=10)
     await _add_bars(session, tid, [(120, 110), (130, 100)], start=date.today())  # run up, pull back
     assert await svc.resolve_open_trades(session) == 1
@@ -236,6 +293,7 @@ async def test_manual_close_sets_reason(session):
 
 
 async def test_list_open_exposes_trailing_stop(session):
+    await svc.set_exit_policy(session, mode="trailing", trailing_pct=12.0)
     tid = await _seed(session, "AAA", close=120.0)
     await _add_open_trade(session, tid, "long", entry=100.0, shares=10, days_ago=10)
     await _add_bars(session, tid, [(125, 118)], start=date.today())  # peak 125