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.

schema.sql

@@ -27,7 +27,8 @@ CREATE TABLE IF NOT EXISTS connections (
     created_at  DATETIME        NOT NULL DEFAULT CURRENT_TIMESTAMP,
     PRIMARY KEY (id),
     KEY idx_node_occurred (node_id, occurred_at),
-    KEY idx_src_ip (src_ip),
+    KEY idx_occurred (occurred_at),
+    KEY idx_src_ip_occ (src_ip, occurred_at),
     KEY idx_dst_port (dst_port),
     CONSTRAINT fk_conn_node FOREIGN KEY (node_id) REFERENCES nodes (id) ON DELETE CASCADE
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;