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/20260316000000-create-buys.up.sql

@@ -0,0 +1,7 @@
+CREATE TABLE buys (
+  id          BIGSERIAL PRIMARY KEY,
+  event_id    BIGINT NOT NULL REFERENCES events(id) UNIQUE,
+  occurred_at DATE NOT NULL,
+  wallet      TEXT NOT NULL,
+  amount_eur  NUMERIC(18,2) NOT NULL
+);