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>

tests/Unit/Offering/OfferingRepositoryTest.php

@@ -35,7 +35,7 @@ class OfferingRepositoryTest extends TestCase
                     return $data['instructor_id'] === 5
                         && $data['kind'] === Offering::KIND_PRIVATE_LESSON
                         && $data['title'] === '30-min Piano'
-                        && $data['price_cents'] === 3500
+                        && $data['price'] === 35.00
                         && $data['allow_weekly'] === 0
                         && $data['is_active'] === 1
                         && $data['created_at'] === '2026-04-01 12:00:00';
@@ -49,7 +49,7 @@ class OfferingRepositoryTest extends TestCase
             instructorId:    5,
             kind:            Offering::KIND_PRIVATE_LESSON,
             title:           '30-min Piano',
-            priceCents:      3500,
+            price:           35.00,
             durationMinutes: 30,
         );
 
@@ -168,7 +168,7 @@ class OfferingRepositoryTest extends TestCase
             'title'            => '30-min Piano',
             'description'      => null,
             'duration_minutes' => '30',
-            'price_cents'      => '3500',
+            'price'            => '35.00',
             'currency'         => 'CAD',
             'billing_mode'     => Offering::BILLING_ONE_TIME,
             'allow_weekly'     => '0',