Security fixes: CSV injection, policy body output, invite hashing, slot datetimes

Four fixes from a security review pass:

- Neutralise CSV formula injection in the payments export: fields with a
  leading =, +, -, @, tab, or CR (e.g. a hostile student display name) are
  apostrophe-prefixed in PaymentReport::csvLine() so they open as text in
  Excel/Google Sheets. Fixes #39.
- Sanitise policy bodies with wp_kses_post at output in
  PolicyEndpoint::index() (the booking JS renders that HTML raw), so a
  future write path that forgets kses can never become stored XSS.
  Fixes #40.
- Store invite tokens hashed (SHA-256) at rest: a database leak can no
  longer redeem pending invites. The registration link is shown once, at
  creation; the pending list shows email/invited date; lookups hash the
  submitted token. Existing plaintext pending invites must be re-issued.
  Fixes #41.
- Validate availability slot datetimes on both creation paths (REST and
  admin form) via AvailabilitySlot::normalizeDateTime(): canonical and
  datetime-local forms normalise to Y-m-d H:i:s, garbage and end <= start
  are rejected (REST 400) instead of reaching the DATETIME column or
  throwing inside the weekly-series date arithmetic. Fixes #42.

composer test (204 tests, 594 assertions), PHPStan L6, and PHPCS all green.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/Unit/Availability/AvailabilitySlotTest.php

@@ -64,6 +64,26 @@ class AvailabilitySlotTest extends TestCase
         self::assertTrue($slot->isBooked);
     }
 
+    public function testNormalizeDateTimeAcceptsCanonicalAndDatetimeLocalForms(): void
+    {
+        self::assertSame('2026-04-01 09:00:00', AvailabilitySlot::normalizeDateTime('2026-04-01 09:00:00'));
+        self::assertSame('2026-04-01 09:00:00', AvailabilitySlot::normalizeDateTime('2026-04-01 09:00'));
+        self::assertSame('2026-04-01 09:00:00', AvailabilitySlot::normalizeDateTime('2026-04-01T09:00'));
+        self::assertSame('2026-04-01 09:00:30', AvailabilitySlot::normalizeDateTime('2026-04-01T09:00:30'));
+    }
+
+    public function testNormalizeDateTimeRejectsGarbageAndImpossibleDates(): void
+    {
+        self::assertNull(AvailabilitySlot::normalizeDateTime(''));
+        self::assertNull(AvailabilitySlot::normalizeDateTime('not a date'));
+        self::assertNull(AvailabilitySlot::normalizeDateTime('next tuesday'));
+        self::assertNull(AvailabilitySlot::normalizeDateTime('2026-04-01'));
+        self::assertNull(AvailabilitySlot::normalizeDateTime('2026-13-01 09:00:00'));
+        self::assertNull(AvailabilitySlot::normalizeDateTime('2026-02-30 09:00:00'));
+        self::assertNull(AvailabilitySlot::normalizeDateTime('2026-04-01 25:00:00'));
+        self::assertNull(AvailabilitySlot::normalizeDateTime("2026-04-01 09:00:00'); DROP TABLE x;--"));
+    }
+
     public function testToArrayContainsExpectedKeys(): void
     {
         $slot = new AvailabilitySlot(1, '2026-04-01 09:00:00', '2026-04-01 10:00:00', 30, 8, false, null, 10);