Line items correctly snapshot both unit_price_cents and
pfand_cents_per_unit, so a later price change doesn't rewrite history.
Returns didn't get the same treatment: transactions.pfand_returns
stored a bare count, and the euro value came from the bar's
pfand_cents at that moment but was never recorded — change a bar's
deposit mid-event and no historical refund amount could be
recomputed from the database.
Writing returns as transaction_items rows (the schema's is_return
column) doesn't fit cleanly: a Pfand return isn't tied to a specific
drink, but transaction_items.drink_id is NOT NULL. Add
transactions.pfand_cents_at_sale instead — same snapshot idea, at
the transaction level where pfand_returns already lives. Also added
to the transactions CSV export so the recovered value is actually
visible somewhere in the app, not just reachable via raw SQL.
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