fix(critical): remove dead manifold.get_probability() from legacy scan and fix BUY_NO payout calculation

- Legacy scan called ManifoldClient.get_probability(), removed in the v3
  matcher migration, causing AttributeError when positions had changed
  family keys. The block used Manifold to escalate positions to
  CLOSE_RECOMMENDED (inversion detection) — a trading decision forbidden
  under MANIFOLD_SIGNAL_ENABLED=false — so the dependency is removed
  entirely; the scan keeps family re-keying and sibling-conflict logic.

- PaperExecutor.close_position() computed cash += position_cost * resolution,
  ignoring direction: a winning BUY_NO (resolution=0.0) paid out $0 and
  reported a loss. Now settles per trade:
    BUY_YES: payout = shares * resolution
    BUY_NO:  payout = shares * (1 - resolution)
  with pnl = payout - net_cost; Telegram win/loss keys off pnl > 0.
  Adds read-only Database.get_open_trades_for_market().

- tests/test_paper_close.py covers the 4 deterministic payout cases;
  tests/conftest.py shims datetime.UTC for local Python 3.10 (prod is 3.11).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/conftest.py

@@ -0,0 +1,10 @@
+"""Test environment shims.
+
+The bot runs on python:3.11-slim in production; local dev machines may have
+3.10, which lacks datetime.UTC (added in 3.11). Alias it so modules using
+`from datetime import UTC` import cleanly under 3.10.
+"""
+import datetime
+
+if not hasattr(datetime, "UTC"):
+    datetime.UTC = datetime.timezone.utc