perf: add occurred_at indexes to connections

Time-window queries (dashboard stats, recent connections, JSON API,
purge) had no usable index and full-scanned the connections table on
every load, so the dashboard slowed down as data accumulated.

- Add idx_occurred (occurred_at) for counts, MAX, ORDER BY ... LIMIT,
  and the purge delete
- Add idx_src_ip_occ (src_ip, occurred_at) so frequent_ips() runs as a
  covering index scan
- Drop the now-redundant standalone idx_src_ip

Migrations in setup.php are idempotent (information_schema checks) and
portable across MySQL 8 and MariaDB.

setup.php

@@ -41,6 +41,43 @@ if ($row > 0) {
     echo "Dropped legacy uq_event unique key.\n";
 }
 
+// Migration: add occurred_at index if upgrading from an older schema.
+// Time-window queries (dashboard, JSON API, purge) full-scanned the table
+// without it, so page loads slowed down as the table grew.
+$row = $pdo->query(
+    "SELECT COUNT(*) FROM information_schema.STATISTICS
+     WHERE TABLE_SCHEMA = DATABASE()
+       AND TABLE_NAME   = 'connections'
+       AND INDEX_NAME   = 'idx_occurred'"
+)->fetchColumn();
+if ($row == 0) {
+    $pdo->exec("ALTER TABLE connections ADD KEY idx_occurred (occurred_at)");
+    echo "Added idx_occurred index.\n";
+}
+
+// Migration: replace the standalone src_ip index with (src_ip, occurred_at)
+// so frequent_ips() resolves as a covering index scan.
+$row = $pdo->query(
+    "SELECT COUNT(*) FROM information_schema.STATISTICS
+     WHERE TABLE_SCHEMA = DATABASE()
+       AND TABLE_NAME   = 'connections'
+       AND INDEX_NAME   = 'idx_src_ip_occ'"
+)->fetchColumn();
+if ($row == 0) {
+    $pdo->exec("ALTER TABLE connections ADD KEY idx_src_ip_occ (src_ip, occurred_at)");
+    echo "Added idx_src_ip_occ index.\n";
+}
+$row = $pdo->query(
+    "SELECT COUNT(*) FROM information_schema.STATISTICS
+     WHERE TABLE_SCHEMA = DATABASE()
+       AND TABLE_NAME   = 'connections'
+       AND INDEX_NAME   = 'idx_src_ip'"
+)->fetchColumn();
+if ($row > 0) {
+    $pdo->exec("ALTER TABLE connections DROP INDEX idx_src_ip");
+    echo "Dropped redundant idx_src_ip index.\n";
+}
+
 // Migration: create settings table and seed default retention if upgrading
 $pdo->exec(
     "CREATE TABLE IF NOT EXISTS settings (