Add availability calendar, admin portal, and booking backend

- db.php: shared config, PDO, SendGrid, package definitions
- availability.php: GET endpoint returning booked/blocked dates by month
- contact.php: booking handler with DB record, availability check, SendGrid emails
- admin/index.php: full admin portal (login, bookings table, status/notes AJAX, block dates)
- index.html: interactive availability calendar with click-to-select, wires to /contact.php
- .htaccess: block direct access to db.php

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

availability.php

@@ -0,0 +1,46 @@
+<?php
+require_once __DIR__ . '/db.php';
+
+header('Content-Type: application/json');
+header('Access-Control-Allow-Origin: https://parkerslingshotrentals.com');
+
+$month = (int)($_GET['month'] ?? date('n'));
+$year  = (int)($_GET['year']  ?? date('Y'));
+$month = max(1, min(12, $month));
+$year  = max(date('Y'), min(date('Y') + 2, $year));
+
+$start = sprintf('%04d-%02d-01', $year, $month);
+$end   = date('Y-m-t', strtotime($start));
+
+// Booked dates from confirmed/pending bookings
+$booked = db()->prepare(
+    "SELECT rental_date, end_date FROM bookings
+     WHERE status IN ('pending','confirmed')
+     AND rental_date <= ? AND end_date >= ?"
+);
+$booked->execute([$end, $start]);
+
+$bookedDays = [];
+foreach ($booked->fetchAll() as $row) {
+    $d = new DateTime($row['rental_date']);
+    $e = new DateTime($row['end_date']);
+    while ($d <= $e) {
+        $bookedDays[] = $d->format('Y-m-d');
+        $d->modify('+1 day');
+    }
+}
+
+// Admin-blocked dates
+$blocked = db()->prepare(
+    "SELECT block_date FROM blocked_dates WHERE block_date BETWEEN ? AND ?"
+);
+$blocked->execute([$start, $end]);
+foreach ($blocked->fetchAll() as $row) {
+    $bookedDays[] = $row['block_date'];
+}
+
+echo json_encode([
+    'month'       => $month,
+    'year'        => $year,
+    'booked_dates' => array_values(array_unique($bookedDays)),
+]);