The layering rule ("request-only logic in routes/; scheduler+worker-shared logic
in the plex|downloads|sync packages; cross-cutting primitives as flat core
modules; extract at the SECOND consumer") was real but unwritten — it lived only
in three module docstrings and a one-line README mention. Collect it, plus the
transaction-ownership rule (a mid-operation helper stages, the caller commits),
the atomic-counter / JobStatus-StrEnum conventions, and the one-home-per-config
rule, into backend/app/README.md. Add docs/development.md (run locally + the gate
commands + branching), so a new session/contributor has one place to look instead
of copying whichever pattern they happen to find first.
