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/Admin/passenger/admin_delete_and_blacklist_passenger.php

@@ -54,5 +54,5 @@ try {
     jsonSuccess(null, "Passenger deleted and blacklisted");
 } catch (Throwable $e) {
     $con->rollBack();
-    jsonError("Failed: ".$e->getMessage());
+    jsonError("An internal error occurred. Please try again later.");
 }