Batch the audit log retention DELETE to bound lock hold time (#66)

deleteOldLogs() ran a single unbounded DELETE. created_at is indexed
so row selection itself is cheap, but on a large qualifying set (first
run after enabling/changing AUDIT_LOG_RETENTION_DAYS, or after the
cron silently missed runs) an unbounded single-statement DELETE holds
row locks for the full duration — risking contention with the frequent
concurrent INSERTs the audit log receives from live traffic. Now
deletes in batches of 1000 (parameterized), looping until nothing
qualifies.

Verified against a local MariaDB instance with a batch size of 10
forcing multiple loop iterations: deleted exactly the stale rows,
left recent rows untouched, correct total count returned.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MGDKHiU5RJdo3dqQUDow3X

models/AuditLogModel.php

@@ -309,17 +309,28 @@ class AuditLogModel
      * @param int $daysToKeep Number of days of logs to keep
      * @return int Number of deleted records
      */
-    public function deleteOldLogs($daysToKeep = 90)
+    public function deleteOldLogs($daysToKeep = 90, $batchSize = 1000)
     {
+        // Batched to bound how long each statement holds row locks — an
+        // unbounded single DELETE on a large backlog (e.g. the first run after
+        // enabling/changing retention, or after the cron silently missed runs)
+        // would otherwise contend with the frequent concurrent INSERTs the
+        // audit log receives from live traffic.
         $stmt = $this->conn->prepare(
-            "DELETE FROM audit_log WHERE created_at < DATE_SUB(NOW(), INTERVAL ? DAY)"
+            "DELETE FROM audit_log WHERE created_at < DATE_SUB(NOW(), INTERVAL ? DAY) ORDER BY audit_id LIMIT ?"
         );
-        $stmt->bind_param("i", $daysToKeep);
-        $stmt->execute();
-        $affectedRows = $stmt->affected_rows;
+        $stmt->bind_param("ii", $daysToKeep, $batchSize);
+
+        $totalDeleted = 0;
+        do {
+            $stmt->execute();
+            $affected = $stmt->affected_rows;
+            $totalDeleted += $affected;
+        } while ($affected > 0);
+
         $stmt->close();
 
-        return $affectedRows;
+        return $totalDeleted;
     }
 
     /**