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.
Celes Renata
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