perf: delete purged connections in batches of 5000

The purge ran one DELETE over the whole retention cutoff in a single
transaction, spiking undo/binlog and holding row locks on a table the
fetch cron writes to. Now it loops in batches of 5000 rows with a short
pause between batches, keeping each commit small.

includes/functions.php

@@ -258,15 +258,34 @@ function frequent_ips(int $min_connections, ?string $since = null): array {
 
 /**
  * Delete connections older than retention_days setting.
+ * Rows are removed in batches so each transaction stays small — a single
+ * giant DELETE would lock a large part of the table while the fetch cron
+ * keeps inserting rows.
+ *
  * Returns the number of rows deleted.
  */
 function purge_old_connections(): int {
     $days   = max(1, (int)get_setting('retention_days', '7'));
+    $cutoff = date('Y-m-d H:i:s', time() - $days * 86400);
+
     $s = db()->prepare(
-        'DELETE FROM connections WHERE occurred_at < NOW() - INTERVAL ? DAY'
+        'DELETE FROM connections
+         WHERE occurred_at < ?
+         ORDER BY id
+         LIMIT 5000'
     );
-    $s->execute([$days]);
-    return $s->rowCount();
+
+    $total = 0;
+    do {
+        $s->execute([$cutoff]);
+        $deleted = $s->rowCount();
+        $total += $deleted;
+        if ($deleted > 0) {
+            usleep(100_000); // 100 ms between batches to ease lock pressure
+        }
+    } while ($deleted >= 5000);
+
+    return $total;
 }
 
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