NWP-101: let ops choose export columns and scope

[aalavala]

What changed

Ops can now click Export on /payments and get an options dialog instead of an immediate download: pick which columns ship (card last-four is off by default), choose between the current filter or every payment, and see the row count before downloading. Deselecting every column disables Download instead of producing an empty file. (Two unrelated files also ride along in this branch — docs/architecture-overview.html and its copy under public/ — a codebase-orientation doc from earlier investigation, not part of this ticket.)

How I verified it

npm test:

✓ src/lib/csv.test.ts (12 tests) 13ms
✓ src/lib/money.test.ts (12 tests) 11ms
✓ src/lib/dates.test.ts (7 tests) 12ms

Test Files  3 passed (3)
     Tests  31 passed (31)

Route, via curl against the running dev server:

• No scope/columns params → same plain filename and full column set as before (payments-2026-09-23.csv).
• columns=id,amount&scope=all → 2-column CSV covering every payment, regardless of the on-screen filter.
• columns= (empty) → 400 with {"message":"Select at least one column to export."}.
• status=disputed&scope=filtered → payments-disputed-2026-09-23.csv.
• status=disputed with no scope param → old plain filename, unchanged (backward compatible).

Dialog, checked by hand in the browser on /payments:

• Opened it — "Card last four" unchecked, every other column checked.
• With status=disputed active, toggled scope from "Current filter" to "All payments" and watched the row count go from 33 to 1,658.
• Cleared every column checkbox and confirmed Download swapped from a link to a disabled button.

Acceptance criteria

• Ops can choose which columns are included. Card last-four is off by default.
• Ops can choose scope: current filter or all payments. Current filter is the default, and the row count is visible before download.
• The filename reflects the scope and the date.
• Amounts stay in minor units internally and are formatted once on the way out, with currency in its own column. (Already true before this ticket — preserved, not changed.)
• Deselecting every column disables Download rather than producing an empty file.

Deliberately not done

• No automated test for the new exportFilename(date, scope) token logic or the export route's scope-branching — the ticket's DoD only calls for a unit test on the column serializer, so those two got manual/curl verification only, not a test file.
• No error state in the dialog if the row-count fetch fails — it just shows "Counting rows…" indefinitely rather than a retry or error message.
• No pageSize allowlist entry added to parseFilters — the dialog's count fetch accepts GET /api/payments's default page of rows and reads only .total, rather than requesting a smaller page.
• NWP-102 (the linked ticket) not investigated or touched.