v0.13.13: @newsroom.writer: credit now, validated later

The user keeps the meter is updated from a second voice memo:

• Credit now, validated later. The user's browser adds credit as soon as they come back from Stripe; the admin side, which can be offline, later signs a validation, or marks the payment as not accepted.
• Three states of credit. Unvalidated, validated, and not valid, which would mean an attack or a bug. The product keeps working on unvalidated credit, because on the client it costs us nothing.
• Who are we defending against? Users gaming the client is unlikely in the short term, and easy to detect.
• A vault shared by two parties. Pin its first commit, check every later one, and know who wrote what.
• Where it lives. user.sgit.ai, or billing.sgit.ai so others can use it easily.
• Both diagrams redrawn to show credit added by the user and validated by the admin.