Fix payments sorting lexicographically instead of numerically by amount

[aalavala]

What changed

Sorting payments by amount was comparing amounts as strings instead of numbers, so a shorter amount with a high leading digit (like 994) could rank above a larger amount (like 9873) on both /payments and GET /api/payments?sort=amount. Amounts now sort by actual numeric value.

How I verified it

npx vitest run src/data/queries.test.ts against the old comparator — failed as expected:

✕ sortPayments > orders amounts numerically, not lexicographically
  expected [ 'pay_small', 'pay_large' ] to deeply equal [ 'pay_large', 'pay_small' ]

After the fix, same test passes, and npm test — 4 files, 29 tests, all passing (money/date/csv unaffected, queries.test.ts is new).

Checked live against the running dev server: curl "http://localhost:3000/api/payments?sort=amount&direction=desc" — top 5 are now 47980, 47935, 47880, 47826, 47778 (strictly decreasing; previously three- and four-digit amounts were interleaved out of order).

Acceptance criteria

(No ticket for this — these are the asks from the bug report itself.)

• Find the root cause: file and line, why it produces the result, whether the same pattern exists elsewhere.
• Fix the root cause.
• Add a unit test that fails on the old comparator and passes on the new one.
• Show the top five payments by amount for manual verification.

Deliberately not done

• No ticket filed in docs/tickets/ for this — it was found ad hoc while manually testing the export feature, not from a tracked backlog item.
• Didn't add a lint rule or broader test guard against localeCompare being misapplied to a non-string field elsewhere in the future — this fix is scoped to the one confirmed instance.
• The export route (GET /api/payments/export) calls the same sortPayments, so it's fixed automatically, not as a separate change — worth knowing it's a shared fix surface, not something addressed twice.