Next healthcheck due date, plus a test suite (hc-nkq)

Lets a technician record when a site should next be looked at, and surfaces
that in the dashboard so it can be used for planning.

Server:
- migration 0002 adds healthchecks.next_due, a VARCHAR(10) 'YYYY-MM-DD'
  calendar date rather than a timestamp — it's a diary date with no
  time-of-day, and a timestamp would render as the wrong day off-server.
- Kept per-session rather than on a sites table, so the history of what was
  scheduled when is preserved. A site's current next-due is the value on its
  most recent session: if the latest visit scheduled nothing, the site reads
  as unscheduled rather than showing the just-completed visit as overdue.
- Validate::optionalDate() round-trips through createFromFormat, which
  rejects both '2026-2-3' and '2026-02-30' (silently rolled to March 2nd).
- next_due follows the same missing-vs-null PUT contract as finished_at, and
  lives in its own Store::setNextDue() so finishing or reopening a session
  never disturbs the date and vice versa.
- Dashboard shows it on the site list, the per-site session table and the
  session detail, flagged overdue / today / soon (within a fortnight).

Plugin:
- Date control on both the active and finished panels — the moment you know
  when to return is often wrap-up, after the report is generated.
- Write-through like every other mutation. Bad input is rejected with a
  notice rather than silently clearing an existing date.
- Included in both the Markdown and HTML reports.

Tests (new — tests/, export-ignored from the plugin zip):
- Hand-rolled harness, no composer/PHPUnit, in keeping with the repo.
- 42 tests: plugin-side date parsing units, plus server integration tests
  driven over real HTTP against a temp `php -S` instance with a throwaway
  SQLite DB, so a real server/config.php is never touched.
- Mutation-checked: dropping the array_key_exists guard on PUT fails three
  tests, as intended.

README.md

@@ -16,8 +16,12 @@ Then activate from *Plugins*. Settings appear under *Tools → Site Healthcheck*
 2. Confirm the recovery plugin status panel shows ✓ active.
 3. Click **Start new healthcheck**.
 4. Work through each step card. For each: choose a status (done / skipped / blocked / n/a) and add notes.
-5. Click **Finish & generate report**.
-6. Download the Markdown report or copy it to clipboard.
+5. Set **Next healthcheck due** to when the site should be looked at again. It's
+   stored centrally and shows against the site on the dashboard, flagged when it's
+   within a fortnight or overdue. Settable before or after finishing; leave it
+   empty (or clear it) if nothing is scheduled.
+6. Click **Finish & generate report**.
+7. Download the Markdown report or copy it to clipboard.
 
 One in-progress session per site at a time. Reports are not stored in the database (the plugin is meant to be uninstalled at the end of each engagement) — download them.