Extend availability (durations, weekly recurrence, calendar); price offerings in dollars

Availability (#2):
- us_availability gains offering_id, duration_minutes (default 60), and
  recurrence_group; AvailabilitySlot carries the new fields.
- AvailabilityRepository::createWeeklySeries() generates N weekly rows
  sharing a recurrence_group; findAvailable() filters by offering and
  duration. Date math uses DateTimeImmutable::modify() (the no-debug CI
  regex `dd\(` matches `->add(`).
- REST GET filters by offering_id/duration_minutes; POST accepts
  duration_minutes, offering_id, recurrence (single|weekly) + weeks.
- Admin form adds duration, an offering picker, and one-off/weekly options
  (OfferingRepository wired into AvailabilityController).
- booking.js renders an agenda calendar (slots grouped by day, with
  duration). The richer booking UX lands with the booking-flow work.

Offering price in dollars:
- Switch us_offerings.price_cents (INT) to price DECIMAL(10,2); Offering
  uses float $price. Admin form and REST take dollars.
- Fix a pre-existing misalignment in the Offering insert/update $wpdb
  format arrays (billing_mode/capacity/is_active were mapped to the wrong
  specifiers, which would corrupt values) via a single COLUMN_FORMATS list.

Also bump PHPStan to --memory-limit=1G in the lint script; 128M now
crashes analysis as the codebase has grown.

Refs #2

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/Schema.php

@@ -13,15 +13,20 @@ class Schema {
 	public static function tables( string $prefix, string $charset ): array {
 		return [
 			"CREATE TABLE {$prefix}us_availability (
-                id           BIGINT UNSIGNED NOT NULL AUTO_INCREMENT,
-                instructor_id BIGINT UNSIGNED NOT NULL,
-                start_dt     DATETIME        NOT NULL,
-                end_dt       DATETIME        NOT NULL,
-                is_booked    TINYINT(1)      NOT NULL DEFAULT 0,
-                created_at   DATETIME        NOT NULL,
+                id               BIGINT UNSIGNED NOT NULL AUTO_INCREMENT,
+                instructor_id    BIGINT UNSIGNED NOT NULL,
+                offering_id      BIGINT UNSIGNED DEFAULT NULL,
+                start_dt         DATETIME        NOT NULL,
+                end_dt           DATETIME        NOT NULL,
+                duration_minutes SMALLINT UNSIGNED NOT NULL DEFAULT 60,
+                is_booked        TINYINT(1)      NOT NULL DEFAULT 0,
+                recurrence_group BIGINT UNSIGNED DEFAULT NULL,
+                created_at       DATETIME        NOT NULL,
                 PRIMARY KEY (id),
                 KEY instructor_id (instructor_id),
-                KEY start_dt (start_dt)
+                KEY offering_id (offering_id),
+                KEY start_dt (start_dt),
+                KEY recurrence_group (recurrence_group)
             ) {$charset};",
 
 			"CREATE TABLE {$prefix}us_lessons (
@@ -45,7 +50,7 @@ class Schema {
                 title            VARCHAR(191)    NOT NULL,
                 description      TEXT,
                 duration_minutes SMALLINT UNSIGNED DEFAULT NULL,
-                price_cents      INT UNSIGNED    NOT NULL DEFAULT 0,
+                price            DECIMAL(10,2)   NOT NULL DEFAULT 0,
                 currency         VARCHAR(3)      NOT NULL DEFAULT 'CAD',
                 billing_mode     VARCHAR(20)     NOT NULL DEFAULT 'one_time',
                 allow_weekly     TINYINT(1)      NOT NULL DEFAULT 0,