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

campsites.php

@@ -3,7 +3,9 @@ $headerStyle = 'light';
 include_once('header.php');
 
 $conn = openDatabaseConnection();
-$result = $conn->query("SELECT * FROM campsites");
+$stmt = $conn->prepare("SELECT * FROM campsites");
+$stmt->execute();
+$result = $stmt->get_result();
 $campsites = [];
 while ($row = $result->fetch_assoc()) {
     $campsites[] = $row;