Mark a matter as one of the firm's own, or clear the mark (admin)

The firm's internal matters are listed in a section of their own in every matter request, whatever the screen shows. This is the one write of firm_internal: no other body takes it and no sync of a practice system changes it. A change raises updated_at, so every device's next pull carries it; the value the matter already has changes nothing. A matter the caller is not served by GET /v1/matters is a 404, as the patch's. The firm marks at most 10 active matters: marking one more is 409 urn:six-minute:problem:list-full and writes nothing; clearing a mark is never refused.