Fix #18: Exception leak remediation across 87 PHP files

- Replaced all client-facing $e->getMessage() with generic error messages
- Added error_log() with filename prefix to all catch blocks
- Covered jsonError(), echo, and json_encode() response patterns
- Also fixed 2 remaining display_errors=1 and add_invoice.php leak
- Script-assisted fix for 75 files, manual fix for 12 remaining edge cases

backend/ride/invitor/get_unified_code.php

@@ -35,7 +35,8 @@ if ($stmt->rowCount() > 0) {
         $insertStmt->execute([$user_id, $role, $newCode]);
         printSuccess(["referral_code" => $newCode]);
     } catch (PDOException $e) {
-        jsonError("Database error: " . $e->getMessage());
+        error_log("[get_unified_code.php] " . $e->getMessage());
+        jsonError("An internal error occurred. Please try again later.");
     }
 }
 ?>