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/facebook.php

@@ -31,7 +31,9 @@ try {
         echo "The token does not have an expiration time.\n";
     }
 } catch (Facebook\Exceptions\FacebookResponseException $e) {
-    echo 'Graph API Error: ' . $e->getMessage();
+    error_log("[facebook.php] Graph API Error: " . $e->getMessage());
+    echo 'An error occurred while fetching Facebook data';
 } catch (Facebook\Exceptions\FacebookSDKException $e) {
-    echo 'SDK Error: ' . $e->getMessage();
+    error_log("[facebook.php] SDK Error: " . $e->getMessage());
+    echo 'An error occurred while processing Facebook data';
 }