fix: position sync now reconciles — removes positions broker no longer holds

The sync_positions loop only upserted positions from Alpaca but never
deleted DB rows for positions that were closed/liquidated on the broker
side. After a paper reset, the next sync would not remove the stale
positions because they simply weren't in Alpaca's response anymore.

Now performs full reconciliation: after upserting what Alpaca reports,
deletes any DB positions for the account that Alpaca no longer holds.

services/adapters/broker_service.py

@@ -428,10 +428,16 @@ async def sync_positions(
     account_uuid: str,
     minio_client: Any | None = None,
 ) -> None:
-    """Sync current positions from Alpaca to PostgreSQL and publish to lake."""
+    """Sync current positions from Alpaca to PostgreSQL and publish to lake.
+
+    Performs a full reconciliation: upserts positions that Alpaca reports,
+    then removes any DB positions that Alpaca no longer holds (e.g. after
+    a paper reset or full liquidation).
+    """
     now = datetime.now(timezone.utc)
     try:
         positions = await adapter.get_positions()
+        broker_tickers = {pos.ticker for pos in positions}
         async with pool.acquire() as conn:
             for pos in positions:
                 await conn.execute(
@@ -444,7 +450,20 @@ async def sync_positions(
                     pos.unrealized_pnl,
                     now,
                 )
-        logger.info("Synced %d positions from Alpaca", len(positions))
+            # Remove positions that the broker no longer reports (closed/liquidated)
+            if broker_tickers:
+                await conn.execute(
+                    "DELETE FROM positions WHERE broker_account_id = $1::uuid AND ticker != ALL($2::varchar[])",
+                    account_uuid,
+                    list(broker_tickers),
+                )
+            else:
+                # Broker reports zero positions — clear all local positions for this account
+                await conn.execute(
+                    "DELETE FROM positions WHERE broker_account_id = $1::uuid",
+                    account_uuid,
+                )
+        logger.info("Synced %d positions from Alpaca (reconciled)", len(positions))
         POSITIONS_SYNCED.inc()
 
         # Publish positions snapshot to analytical lake