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>

templates/admin/offerings.php

@@ -35,8 +35,8 @@ if (! defined('ABSPATH')) {
                 <td><input type="number" name="duration_minutes" id="duration_minutes" min="0" step="1"></td>
             </tr>
             <tr>
-                <th><label for="price_cents"><?php esc_html_e('Price (cents)', 'unsupervised-schedular'); ?></label></th>
-                <td><input type="number" name="price_cents" id="price_cents" min="0" step="1" value="0"></td>
+                <th><label for="price"><?php esc_html_e('Price (dollars)', 'unsupervised-schedular'); ?></label></th>
+                <td><input type="number" name="price" id="price" min="0" step="0.01" value="0.00"></td>
             </tr>
             <tr>
                 <th><label for="billing_mode"><?php esc_html_e('Billing', 'unsupervised-schedular'); ?></label></th>
@@ -86,7 +86,7 @@ if (! defined('ABSPATH')) {
                         <td><?php echo esc_html($offering->title); ?></td>
                         <td><?php echo esc_html($offering->kind); ?></td>
                         <td><?php echo $offering->durationMinutes ? esc_html((string) $offering->durationMinutes . ' min') : '&mdash;'; ?></td>
-                        <td><?php echo esc_html(number_format($offering->priceCents / 100, 2) . ' ' . $offering->currency); ?></td>
+                        <td><?php echo esc_html(number_format($offering->price, 2) . ' ' . $offering->currency); ?></td>
                         <td><?php echo esc_html($offering->billingMode); ?></td>
                         <td><?php echo $offering->isActive ? esc_html__('Yes', 'unsupervised-schedular') : esc_html__('No', 'unsupervised-schedular'); ?></td>
                         <td>