feat(scan): legacy position scan — re-key, Manifold re-validate, auto-close

Adds run_legacy_scan() that executes once at startup before the trading loop:

  1. Re-keys every open DB position using the current market_family_key()
  2. Groups by new family key; KEEP = highest edge_net, CLOSE_RECOMMENDED = sibling
  3. Manifold re-query for positions whose family key changed; if corrected
     probability contradicts the trade direction → CLOSE_RECOMMENDED
  4. Logs full report (KEEP / REVIEW / CLOSE_RECOMMENDED) before any closures
  5. In paper mode: auto-closes all CLOSE_RECOMMENDED positions

For the existing Ohio bug:
  - Democrats win Ohio governor (629557): CLOSE_RECOMMENDED
    family changed ohio-democrat-2026 → ohio-gubernatorial-2026
    Manifold re-query confirms prob=0.05 contradicts BUY_YES (inversion bug)
    $X returned to cash at break-even
  - Republicans win Ohio governor (629558): KEEP
    higher edge_net (0.349 > 0.247)

Infrastructure:
  - schema.sql: closed_at TIMESTAMPTZ, close_reason TEXT on trades
  - db.py: all open-position queries filter WHERE closed_at IS NULL
           + close_paper_position(market_id, reason)
  - paper.py: close_legacy_position(market_id, reason) → float

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

bot/executor/paper.py

@@ -159,6 +159,23 @@ class PaperExecutor:
 
         return trade
 
+    async def close_legacy_position(self, market_id: str, reason: str) -> float:
+        """
+        Close a paper position flagged by the legacy scan.
+
+        Returns the capital recovered to cash (cost basis, assuming break-even
+        exit — exact P&L would require the live exit price which isn't available
+        at scan time).
+        """
+        cost = self._portfolio.positions.pop(market_id, 0.0)
+        self._portfolio.cash += cost  # return capital at break-even
+        await self._db.close_paper_position(market_id, reason)
+        log.warning(
+            "LEGACY_CLOSE market=%s | returned $%.2f to cash | %s",
+            market_id, cost, reason[:80],
+        )
+        return cost
+
     async def close_position(self, market_id: str, resolution: float) -> Optional[float]:
         """
         Close a paper position after market resolution.