Stop the availability form failing in silence

Adding availability for 5:30-6:00 PM with the lesson length left on its
60-minute default saved nothing and said nothing. A window is stored as
consecutive lesson-length slots, so one that fits no lesson splits into
none: splitByDuration() returned [], createFromWindow() inserted
nothing, and addSlot() discarded the result and re-rendered the page
unchanged.

The REST endpoint already rejected that window with a 400. The admin
form checked the same rules separately, and its copy was both laxer and
mute — an unreadable date, an end before the start, and a two-day window
were bare `return`s, and it never checked offering ownership at all, so
a crafted POST could tie a slot to another instructor's offering and
inherit their price and payment routing.

Both callers now go through WindowValidator, which returns the window or
a WP_Error explaining the refusal. The endpoint returns that error as
is; the page renders its message as a notice. handleFormAction returns
a [notice, error] pair so deletes report themselves too, and a
successful add says how many slots it created.

Two failures could also go unnoticed underneath: wpdb::insert's result
was ignored, and insert_id still holds the previous statement's id after
a failed write, so a failure looked like a success — and could become
the recurrence group of a weekly series, orphaning every later
occurrence. weeks was unbounded server-side despite the form's max=52.

availability-admin.js narrows the lesson-length choices to those that
fit the window and blocks submission when none do, which is what makes
the original mistake hard to repeat. It is a convenience: the server
validates regardless.

Closes #130

src/Availability/AvailabilityRepository.php

@@ -11,8 +11,14 @@ class AvailabilityRepository {
 		$this->table = $db->prefix . 'us_availability';
 	}
 
+	/**
+	 * Insert one slot row. Returns its id, or 0 when the write failed —
+	 * `insert_id` still holds the *previous* statement's id after a failed
+	 * insert, so returning it unconditionally made a failed write look like a
+	 * successful one.
+	 */
 	public function insert( AvailabilitySlot $slot ): int {
-		$this->db->insert(
+		$written = $this->db->insert(
 			$this->table,
 			[
 				'instructor_id'    => $slot->instructorId,
@@ -27,7 +33,7 @@ class AvailabilityRepository {
 			[ '%d', '%d', '%s', '%s', '%d', '%d', '%d', '%s' ]
 		);
 
-		return $this->db->insert_id;
+		return false === $written ? 0 : $this->db->insert_id;
 	}
 
 	/**
@@ -42,9 +48,17 @@ class AvailabilityRepository {
 		$ids = [];
 
 		foreach ( $window->splitByDuration() as $slot ) {
-			$ids = $weekly
-				? array_merge( $ids, $this->createWeeklySeries( $slot, $weeks ) )
-				: [ ...$ids, $this->insert( $slot ) ];
+			if ( $weekly ) {
+				$ids = array_merge( $ids, $this->createWeeklySeries( $slot, $weeks ) );
+				continue;
+			}
+
+			$id = $this->insert( $slot );
+
+			// A failed insert returns 0; it must not reach the caller as an id.
+			if ( $id > 0 ) {
+				$ids[] = $id;
+			}
 		}
 
 		return $ids;
@@ -55,10 +69,14 @@ class AvailabilityRepository {
 	 * separate row one week apart, all sharing a `recurrence_group` (the id of the
 	 * first row).
 	 *
+	 * The count is clamped to `AvailabilitySlot::MAX_WEEKLY_OCCURRENCES`. The
+	 * form's `max` attribute says the same, but only this is binding — a
+	 * hand-crafted POST used to be able to ask for an unbounded number of rows.
+	 *
 	 * @return list<int> Inserted slot IDs.
 	 */
 	public function createWeeklySeries( AvailabilitySlot $first, int $occurrences ): array {
-		$occurrences = max( 1, $occurrences );
+		$occurrences = max( 1, min( AvailabilitySlot::MAX_WEEKLY_OCCURRENCES, $occurrences ) );
 		$start       = new \DateTimeImmutable( $first->startDt );
 		$end         = new \DateTimeImmutable( $first->endDt );
 
@@ -79,6 +97,13 @@ class AvailabilityRepository {
 				)
 			);
 
+			// A failed insert returns 0. Skipping it keeps a bogus id out of the
+			// returned list and, more importantly, stops 0 becoming the series'
+			// recurrence group — which would orphan every later occurrence.
+			if ( $id <= 0 ) {
+				continue;
+			}
+
 			if ( 0 === $groupId ) {
 				$groupId = $id;
 				$this->setRecurrenceGroup( $id, $groupId );