Add buys query table from exchange_to_wallet events

Denormalized read table with id, event_id, occurred_at, wallet, sats,
fee_sats, and amount_eur. Projected on event creation and rebuildable
via POST /api/buys/rebuild. Fiat amounts converted to EUR using
Frankfurter rates.

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

resources/migrations/20260316100000-buys-add-sats.up.sql

@@ -0,0 +1,3 @@
+ALTER TABLE buys ADD COLUMN sats BIGINT NOT NULL DEFAULT 0;
+--;;
+ALTER TABLE buys ADD COLUMN fee_sats BIGINT;