fix: address PR review findings — error handling and safety

Fixes from code review and silent failure audit:

- SnapshotRestoreEngine: catch only duplicate key errors (MySQL 1062)
  in merge mode, re-throw all other exceptions instead of swallowing
- SnapshotRestoreEngine: add json_last_error() check for better error
  messages on corrupt snapshot files
- SnapshotRestoreEngine: log warnings when set_time_limit/ini_set fail
- SnapshotEngine: use strlen($json) instead of filesize() to avoid
  race conditions; catch \Exception instead of \Throwable
- SnapshotsController: remove @unlink suppression, add try-catch
  around delete loop with partial failure reporting
- script.php: add user-facing warning when webcron secret generation
  fails (was silently swallowed, inconsistent with other catch blocks)

source/script.php

@@ -232,6 +232,11 @@ class Pkg_MokoSuiteBackupInstallerScript
 			$db->execute();
 		} catch (\Exception $e) {
 			error_log('MokoSuiteBackup: generateWebcronSecret() failed: ' . $e->getMessage());
+			Factory::getApplication()->enqueueMessage(
+				'MokoSuiteBackup could not generate a random webcron secret. '
+				. 'Please set one manually in the component options to secure the webcron endpoint.',
+				'warning'
+			);
 		}
 	}