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>

db.php

@@ -0,0 +1,56 @@
+<?php
+define('PARKER_DB_HOST', 'localhost');
+define('PARKER_DB_NAME', 'parker_db');
+define('PARKER_DB_USER', 'parker_user');
+define('PARKER_DB_PASS', 'Pk4rk3r_2026!Tx');
+
+define('ADMIN_USER', 'admin');
+define('ADMIN_PASS', '$2y$12$Tz8QkXv9mNpL3wR1uE6OaOQbKjH5sYdF2cVnMxPt7lGAeWqZiBSHu'); // Parker2026!
+define('ADMIN_SESSION_KEY', 'parker_admin_auth');
+
+define('SENDGRID_API_KEY', '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');
+define('MAIL_FROM',       'noreply@parkerslingshotrentals.com');
+define('MAIL_FROM_NAME',  'Parker County Slingshot Rentals');
+define('ADMIN_EMAIL',     'info@parkerslingshotrentals.com');
+
+define('PACKAGES', [
+    'half-day' => ['label' => 'Half Day (4 hrs)',    'amount' => 99.00,  'days' => 0],
+    'full-day' => ['label' => 'Full Day (8 hrs)',     'amount' => 169.00, 'days' => 0],
+    'weekend'  => ['label' => 'Weekend (48 hrs)',     'amount' => 299.00, 'days' => 1],
+]);
+
+function db(): PDO {
+    static $pdo;
+    if (!$pdo) {
+        $pdo = new PDO(
+            'mysql:host=' . PARKER_DB_HOST . ';dbname=' . PARKER_DB_NAME . ';charset=utf8mb4',
+            PARKER_DB_USER, PARKER_DB_PASS,
+            [PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION, PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC]
+        );
+    }
+    return $pdo;
+}
+
+function generateRef(): string {
+    return 'PSR-' . strtoupper(substr(uniqid(), -6));
+}
+
+function sendEmail(string $to, string $toName, string $subject, string $html): bool {
+    if (!SENDGRID_API_KEY || strpos(SENDGRID_API_KEY, 'YOUR_KEY') !== false) return false;
+    $payload = json_encode([
+        'personalizations' => [['to' => [['email' => $to, 'name' => $toName]]]],
+        'from'    => ['email' => MAIL_FROM, 'name' => MAIL_FROM_NAME],
+        'subject' => $subject,
+        'content' => [['type' => 'text/html', 'value' => $html]],
+    ]);
+    $ch = curl_init('https://api.sendgrid.com/v3/mail/send');
+    curl_setopt_array($ch, [
+        CURLOPT_RETURNTRANSFER => true, CURLOPT_POST => true,
+        CURLOPT_POSTFIELDS => $payload,
+        CURLOPT_HTTPHEADER => ['Authorization: Bearer ' . SENDGRID_API_KEY, 'Content-Type: application/json'],
+        CURLOPT_TIMEOUT => 15, CURLOPT_SSL_VERIFYPEER => false,
+    ]);
+    $code = curl_getinfo($ch, CURLINFO_HTTP_CODE);
+    curl_exec($ch); curl_close($ch);
+    return $code === 202;
+}