Fix migration chain and clear stored paper exit mode

Migration 017 set down_revision to "016_add_signal_context_snapshots",
but migration 016's revision id is "016", so `alembic upgrade head` could
not resolve the chain and the deploy's auto-alembic step would fail. Point
017 at the real id "016".

Add migration 018 to delete any persisted `paper_exit_mode` row. The July
2026 promotion changed the paper-trade exit default to `atr_trailing`, but
`get_exit_policy` reads a stored value before the code default, so an
environment that had ever saved the old `time` mode would silently keep it
and never run the promoted 3x ATR trailing exit. Mirrors migration 015's
one-way settings reset.

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

alembic/versions/018_clear_paper_exit_mode.py

@@ -0,0 +1,43 @@
+"""Clear the stored paper exit mode so the promoted ATR-trail default applies.
+
+The July 2026 promotion moved the paper-trade auto-exit default from ``time``
+(30-day hold on the initial stop) to ``atr_trailing`` (initial stop + 3x ATR
+trailing stop, max 30 trading days) — the exit the production backtest
+validated, now implemented live in ``paper_trade_service``.
+
+``get_exit_policy`` reads a stored ``paper_exit_mode`` row before falling back
+to the code default, so any environment that persisted the old ``time`` value
+(e.g. via Admin -> Paper-Trade Exit, or an earlier deploy) would silently keep
+the old exit and never run the promoted ATR trail. Following the precedent of
+migration 015, the stored row is cleared here so the new code default takes
+effect. Admins can re-tune in Admin -> Paper-Trade Exit if desired. Note: this
+changes how open paper trades close, so Track Record comparability resets from
+this deploy.
+
+Revision ID: 018_clear_paper_exit_mode
+Revises: 017_add_trade_setup_strategy_rank
+Create Date: 2026-07-04 00:00:00.000000
+"""
+
+from __future__ import annotations
+
+from alembic import op
+import sqlalchemy as sa
+
+
+revision = "018_clear_paper_exit_mode"
+down_revision = "017_add_trade_setup_strategy_rank"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.execute(
+        sa.text("DELETE FROM system_settings WHERE key = 'paper_exit_mode'")
+    )
+
+
+def downgrade() -> None:
+    # One-way data reset: the prior stored value isn't recoverable. The code
+    # default applies until re-tuned, so there is nothing to restore.
+    pass