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/v2/financial/settlements.php

@@ -39,6 +39,7 @@ try {
     ]);
 } catch (Exception $e) {
     http_response_code(500);
-    echo json_encode(['status' => 'error', 'message' => $e->getMessage()]);
+    error_log("[settlements.php] " . $e->getMessage());
+    echo json_encode(['status' => 'error', 'message' => 'An internal error occurred']);
 }
 ?>