fix(api): marketplace listing should reject expired RIs and return 4xx (not 500) for bad timestamps/terms #449

Follow-ups from the review of #448 (closes #98):

• internal/api/handler_marketplace.go ~455-458 and ~625: a fully expired RI still lists at 1 month of residual value (the floor-at-1 guard). AWS rejects retired RIs anyway; reject expired RIs up front with a 4xx.
• ~150-153: zero-timestamp and invalid-term errors are plain fmt.Errorf values, so the client gets a generic 500. Wrap them as NewClientError(422, ...) so the user sees why. It fails closed today (no listing row and no EC2 call happen).
• Nit: the invalid-term message changed wording in fix(api): refuse marketplace pricing for an RI with no purchase timestamp #448; consider restoring 'expected 1 or 3 years'.