Fix: Use absolute paths for all upload directories in processor files

- upload_profile_picture.php: Use absolute path for profile picture uploads, store relative path in DB
- submit_pop.php: Use absolute path for proof of payment uploads
- process_signature.php: Use absolute path for signature uploads, store relative path in DB

src/processors/submit_pop.php

@@ -34,7 +34,7 @@ if ($_SERVER['REQUEST_METHOD'] === 'POST') {
         exit;
     }
     
-    $target_dir = "uploads/pop/";
+    $target_dir = $rootPath . "/src/processors/uploads/pop/";
     $randomFilename = $validationResult['filename'];
     $target_file = $target_dir . $randomFilename;