perf(api): simplify hourly balance history query

[raymondjacobson]

Summary

• uses a direct grouped query for hourly balance history responses
• keeps daily balance history on the existing end-of-day semantics, but removes the runtime granularity CASE
• removes the unused @granularity bind from this endpoint query

Production evidence

• the database audit found /v1/users/:id/balance/history paying for the daily bucketing/DISTINCT layer even when the default granularity=hourly path only needs hourly totals
• hourly callers now use the covered user_id/timestamp predicate, GROUP BY timestamp, and ORDER BY timestamp ASC directly
• daily callers still sum mints per hour first and then pick the latest hour per day

Verification

• go test ./api -run 'TestUserBalanceHistory' -count=1 -timeout=2m