Standardize: Convert 5 more queries to prepared statements

Converted queries in:
- functions.php:
  * countUpcomingTrips() - Trip count query
  * getNextOpenDayDate() - Next open day event lookup

- campsites.php:
  * All campsites query for map display

- fetch_users.php:
  * User list query (AJAX handler)

- get_campsites.php:
  * Campsites with user join (AJAX handler)

All now use prepared statements with proper parameter binding.
Progress: 12/21 queries converted. Remaining: fetch_drinks, fetch_bar_tabs, admin pages (legacy_members queries), bush_mechanics course query

get_campsites.php

@@ -4,15 +4,15 @@ include_once('connection.php');
 include_once('functions.php');
 $conn = openDatabaseConnection();
 
-$sql = "SELECT 
+$stmt = $conn->prepare("SELECT 
             c.*,
             u.first_name,
             u.last_name,
             u.profile_pic
         FROM campsites c
-        LEFT JOIN users u ON c.user_id = u.user_id";
-
-$result = $conn->query($sql);
+        LEFT JOIN users u ON c.user_id = u.user_id");
+$stmt->execute();
+$result = $stmt->get_result();
 
 $campsites = [];
 while ($row = $result->fetch_assoc()) {