GET /v1/payments?state= filter

Support and ops staff investigating "why are so many payments failing" need to filter by state without pulling every payment and filtering client-side. GET /v1/payments always returns the 100 most recently created payments, full stop, and the state-filtered route currently 500s.

GET /v1/payments always returns the 100 most recently created payments, full stop. The handler already parses a ?state= query parameter and routes to payment.Service.ListByState when it's present -- but Service.ListByState just calls Store.ListByState, which is an unimplemented, panicking stub. Right now, GET /v1/payments?state=failed returns 500 Internal Server Error.

go run ./cmd/paystate curl -s "localhost:8080/v1/payments?state=failed"

-> 500 Internal Server Error, panic logged: Store.ListByState not implemented

Support and ops staff investigating "why are so many payments failing" need to filter by state without pulling every payment and filtering client-side. Store.List already shows the query shape (SELECT ... FROM payments ORDER BY created_at DESC LIMIT $1) -- this is the same query with one more predicate.

Look at Store.List right above Store.ListByState in internal/payment/store.go for the query shape and scanPayment usage pattern to copy. You need a WHERE state = $1 clause and to shift the LIMIT parameter accordingly. Don't touch payments_state_check or any migration -- state is already a plain indexed column.