A decision record that recorded a reversal

A record superseding one from 2021, written by the same person, saying the opposite.

# ADR-021: Merge the billing service back

## Status
Accepted, 2024-01-03. Supersedes ADR-009 (2021-04).

## Context
ADR-009 split billing into a separate deployable to
allow independent scaling and deployment. Three years
on: it has never been scaled independently, and it has
never been deployed independently — 41 of 44 releases
touched both.

## What ADR-009 got right
The module boundary. That is kept, enforced by a layer
rule rather than by a network.

The section naming what the superseded record got right is the one that makes this readable rather than defensive, and it is the part that took longest to write. A record that only says the old decision was wrong is a document nobody wants to write, so it does not get written and the reversal happens without a trace.