Resolve an AI incident

Moves the incident to resolved, stamps resolved_at, and records the resolution summary.

resolution is required. This operation is the only writer of resolution_summary on an incident record — PATCH /api/v1/rbi/incidents/{id} does not carry the field — so an incident resolved without one could never acquire it afterwards.

Registered since the module's first release and unserved until #3935: the handler's sub-route switch had no case for resolve, so every request answered 405.