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>

assets/js/booking.js

@@ -30,16 +30,50 @@
         errorBox.style.display = 'block';
     }
 
+    function dayKey(dt) {
+        return String(dt).slice(0, 10);
+    }
+
+    function timeOf(dt) {
+        return String(dt).slice(11, 16);
+    }
+
+    function dayLabel(key) {
+        const date = new Date(key + 'T00:00:00');
+        if (Number.isNaN(date.getTime())) return key;
+        return date.toLocaleDateString(undefined, {
+            weekday: 'long', year: 'numeric', month: 'long', day: 'numeric',
+        });
+    }
+
+    function groupByDay(slots) {
+        const groups = new Map();
+        slots.forEach((slot) => {
+            const key = dayKey(slot.start_dt);
+            if (!groups.has(key)) groups.set(key, []);
+            groups.get(key).push(slot);
+        });
+        return [...groups.entries()].sort((a, b) => a[0].localeCompare(b[0]));
+    }
+
+    // Agenda-style calendar: available slots grouped by day. The richer booking
+    // UX (offering selection, intake questions, policy acceptance) lands with the
+    // booking-flow work.
     function renderSlots(slots) {
         if (!slots.length) {
             slotList.innerHTML = '<p>No available lesson slots at this time.</p>';
             return;
         }
 
-        slotList.innerHTML = slots.map((slot) => `
-            <div class="us-slot">
-                <span>${escHtml(slot.start_dt)} – ${escHtml(slot.end_dt)}</span>
-                <button data-slot-id="${slot.id}" class="us-book-btn">Book</button>
+        slotList.innerHTML = groupByDay(slots).map(([key, daySlots]) => `
+            <div class="us-day">
+                <h3 class="us-day-heading">${escHtml(dayLabel(key))}</h3>
+                ${daySlots.map((slot) => `
+                    <div class="us-slot">
+                        <span>${escHtml(timeOf(slot.start_dt))}–${escHtml(timeOf(slot.end_dt))} (${escHtml(String(slot.duration_minutes))} min)</span>
+                        <button data-slot-id="${slot.id}" class="us-book-btn">Book</button>
+                    </div>
+                `).join('')}
             </div>
         `).join('');