fix(accounting): use size_usdc (not net_cost) for positions in initialize()

Eliminates the phantom 0.4% exposure overage after pod restarts.

During live trading execute() stores size_usdc in portfolio.positions and
deducts net_cost from cash — so total_value = bankroll − fees and
exposure_pct = sum(size_usdc) / (bankroll − fees).

Old initialize() stored net_cost in positions, making total_value = bankroll
and inflating exposure_pct (observed: 30.085% vs runtime 29.670%).

Fix: new get_open_position_data() returns both {market_id: size_usdc} and
total_net_cost in one query; initialize() uses size_usdc for positions and
total_net_cost for cash — identical model to execute().

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

bot/data/db.py

@@ -91,6 +91,27 @@ class Database:
             )
             return {r["market_id"]: float(r["total"]) for r in rows}
 
+    async def get_open_position_data(self) -> tuple[dict[str, float], float]:
+        """Return (positions_by_size_usdc, total_net_cost) for all open trades.
+
+        positions_by_size_usdc  — {market_id: size_usdc} mirrors what live trading
+                                  stores in portfolio.positions (no fee included).
+        total_net_cost          — SUM(net_cost) across all open trades, used to
+                                  reconstruct cash = bankroll − total_net_cost.
+
+        Together these let initialize() replicate the exact same accounting model
+        that execute() uses at runtime, eliminating the phantom exposure overage
+        caused by the old net_cost-in-positions approach.
+        """
+        async with self._pool.acquire() as conn:
+            rows = await conn.fetch(
+                "SELECT market_id, SUM(size_usdc) AS sz, SUM(net_cost) AS nc "
+                "FROM trades WHERE closed_at IS NULL GROUP BY market_id"
+            )
+            positions = {r["market_id"]: float(r["sz"]) for r in rows}
+            total_net_cost = sum(float(r["nc"]) for r in rows)
+            return positions, total_net_cost
+
     async def get_open_families(self) -> set[str]:
         """Return the set of family_key values from all open positions.