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/data/db.py

@@ -65,15 +65,11 @@ class Database:
             )
 
     async def get_open_positions(self) -> dict[str, float]:
-        """Return {market_id: total_net_cost} for all trades in DB.
-
-        Since there is no closed flag, every trade in the DB is treated as an
-        open position.  After a TRUNCATE the query returns nothing, so the
-        portfolio correctly resets to a full bankroll.
-        """
+        """Return {market_id: total_net_cost} for all open (not closed) trades in DB."""
         async with self._pool.acquire() as conn:
             rows = await conn.fetch(
-                "SELECT market_id, SUM(net_cost) AS total FROM trades GROUP BY market_id"
+                "SELECT market_id, SUM(net_cost) AS total "
+                "FROM trades WHERE closed_at IS NULL GROUP BY market_id"
             )
             return {r["market_id"]: float(r["total"]) for r in rows}
 
@@ -85,7 +81,8 @@ class Database:
         """
         async with self._pool.acquire() as conn:
             rows = await conn.fetch(
-                "SELECT DISTINCT family_key FROM trades WHERE family_key IS NOT NULL"
+                "SELECT DISTINCT family_key FROM trades "
+                "WHERE family_key IS NOT NULL AND closed_at IS NULL"
             )
             return {r["family_key"] for r in rows if r["family_key"]}
 
@@ -101,11 +98,20 @@ class Database:
                 SELECT DISTINCT ON (market_id)
                     market_id, question, direction, edge_net, family_key, timestamp
                 FROM trades
-                WHERE paper = TRUE
+                WHERE paper = TRUE AND closed_at IS NULL
                 ORDER BY market_id, timestamp DESC
             """)
             return [dict(r) for r in rows]
 
+    async def close_paper_position(self, market_id: str, reason: str = "") -> None:
+        """Mark a paper position as closed (sets closed_at timestamp)."""
+        async with self._pool.acquire() as conn:
+            await conn.execute(
+                "UPDATE trades SET closed_at = NOW(), close_reason = $2 "
+                "WHERE market_id = $1 AND closed_at IS NULL",
+                market_id, reason,
+            )
+
     async def get_recent_trades(self, limit: int = 100) -> list[dict]:
         async with self._pool.acquire() as conn:
             rows = await conn.fetch(