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/017_add_trade_setup_strategy_rank.py

@@ -1,7 +1,7 @@
 """Add production strategy rank fields to trade setups.
 
 Revision ID: 017_add_trade_setup_strategy_rank
-Revises: 016_add_signal_context_snapshots
+Revises: 016
 Create Date: 2026-07-03 20:15:00.000000
 """
 
@@ -12,7 +12,7 @@ import sqlalchemy as sa
 
 
 revision = "017_add_trade_setup_strategy_rank"
-down_revision = "016_add_signal_context_snapshots"
+down_revision = "016"
 branch_labels = None
 depends_on = None