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>

alembic/versions/015_phase3_exit_and_gate.py

@@ -0,0 +1,50 @@
+"""Phase 3 strategy adoption: time-based exit + confidence floor removed.
+
+The July 2026 backtest (gate ablation graded under both exit models, plus the
+capital-constrained portfolio simulation) concluded:
+
+- The best exit is hold-to-horizon: keep the initial ATR stop and exit at the
+  30th trading day's close (+0.50R net/trade vs +0.13R for the S/R target
+  exit; simulated book +31.9% vs +23.7% CAGR at the same drawdown). The paper
+  trade exit-policy default is now ``time`` (30 trading days).
+- The confidence floor adds nothing (identical net/trade with it removed,
+  under both exit models) while cutting ~25% of qualified trades. Its default
+  is now 0 (off).
+
+Stored rows for these two settings were written under the old semantics, so
+they are cleared here and the new code defaults take effect. Re-tune in
+Admin -> Activation / Paper-Trade Exit if desired. Note: this changes which
+setups qualify and how paper trades close, so Track Record comparability
+resets from this deploy.
+
+Revision ID: 015
+Revises: 014
+Create Date: 2026-07-02 00:00:00.000000
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = "015"
+down_revision: Union[str, None] = "014"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.execute(
+        sa.text(
+            "DELETE FROM system_settings "
+            "WHERE key IN ('activation_min_confidence', 'paper_exit_mode')"
+        )
+    )
+
+
+def downgrade() -> None:
+    # One-way data reset: the old per-key values aren't recoverable. Code
+    # defaults apply until re-tuned, so there is nothing to restore.
+    pass