[kyle-lesinger]

Follow-up to #250. This commit was pushed after #250 had already merged, so it never reached main.

What

A blank end date used to be a hard error ("End date is required.") in both rules.ts and validate_data.py. That blocked filing an event that's still underway. The Disasters Portal itself records only a startDate.

• Rule: now only a malformed or out-of-order end date is an error. Two parity fixtures were added: an ongoing event passes, and a malformed end still fails.
• Display: a missing end date reads as ongoing ("9 Oct 2026 – ongoing").
• Date filter: ongoing events now match any later Events date filter. overlaps() used to treat "" as already ended, which silently hid them.
• Request file: end is omitted rather than written as "".
• Import: it no longer invents end = start, so Hurricane Isaias is now ongoing.
• Form: the field reads "End date (optional)", with the hint "Leave blank while the event is ongoing".

Verified

• validate_data.py and rules_parity_test.py pass, as do npm run typecheck and npm run build.
• scripts/e2e.mjs passes 33/33 in Chromium. It includes three new checks:
  • an ongoing event renders as "– ongoing";
  • it still matches a later date filter;
  • Edit page opens with a blank end date and Open PR emits no end key.