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.

server/migrations/0002_next_due.sql

@@ -0,0 +1,18 @@
+-- Next healthcheck due date, recorded against the session that scheduled it.
+--
+-- Stored as a VARCHAR(10) 'YYYY-MM-DD' calendar date rather than a BIGINT unix
+-- timestamp on purpose: this is a diary date a human picked ("look at this site
+-- again in October"), not an instant. A timestamp would drag timezone handling
+-- into something that has no time-of-day component, and would render as the
+-- wrong day for anyone east or west of the server.
+--
+-- Kept per-session (not on a separate 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 — see Store::allSitesSummary().
+--
+-- Portable: both MySQL 8 and SQLite accept ALTER TABLE ... ADD COLUMN with a
+-- nullable typed column and no default. Neither supports ADD COLUMN IF NOT
+-- EXISTS in a form the other understands, which is fine — Migrations.php only
+-- ever applies each file once.
+
+ALTER TABLE healthchecks ADD COLUMN next_due VARCHAR(10) NULL;