Missing index on growing table

An orders API (Ruby on Rails, Postgres) degrades gradually over two weeks, nobody noticed a single bad deploy. p99 on GET /orders crept from 120ms to 3s. The slow-query log now shows the order-history query (filter by customer_id, order by created_at) doing a sequential scan over the orders table, which has grown from 2M to 30M rows after a successful growth quarter. EXPLAIN ANALYZE confirms a Seq Scan + sort. The table has a primary key on id but no other indexes. Triage and mitigate.