Standardize: Convert 7 high-priority $conn->query() to prepared statements

Converted queries in:
- functions.php:
  * getTripCount() - Hardcoded query
  * getAvailableSpaces() - Two queries using $trip_id parameter (HIGH PRIORITY)

- blog.php:
  * Main blog list query - Hardcoded 'published' status

- course_details.php:
  * Driver training courses query - Hardcoded course type

- driver_training.php:
  * Future driver training dates query - Hardcoded course type

- events.php:
  * Upcoming events query - Hardcoded date comparison

- index.php:
  * Featured trips query - Hardcoded published status

All queries now use proper parameter binding via prepared statements.
Next: Convert remaining 15+ safe hardcoded queries for consistency.

index.php

@@ -83,12 +83,15 @@ if (countUpcomingTrips() > 0) { ?>
             <div class="row justify-content-center">
                 <?php
                 // Query to retrieve data from the trips table
-               $sql = "SELECT trip_id, trip_name, location, short_description, start_date, end_date, vehicle_capacity, cost_members, places_booked 
+               $stmt = $conn->prepare("SELECT trip_id, trip_name, location, short_description, start_date, end_date, vehicle_capacity, cost_members, places_booked 
         FROM trips 
-        WHERE published = 1 
+        WHERE published = ? 
         ORDER BY trip_id DESC 
-        LIMIT 4";
-                $result = $conn->query($sql);
+        LIMIT 4");
+                $published = 1;
+                $stmt->bind_param("i", $published);
+                $stmt->execute();
+                $result = $stmt->get_result();
 
                 if ($result->num_rows > 0) {
                     // Loop through each row