Backend changes

Conventions for API routes, domain code, jobs and errors in msaving/backend.

Where code goes

Layer	Folder	Holds
core	`app/core/`	Settings, sign-in building blocks, session check, per-request database session, account event log, update check
api	`app/api/`	FastAPI app (web app files, CSP, Origin check), dependencies, error mapping, models, `routes/v1/`
domain	`app/domain/`	Logic used by routes: transactions, dashboard, export, starter categories
jobs	`app/jobs/`	Migrations runner, scheduler, daily jobs (`python -m app.jobs.<name>`)
	`app/launch.py`, `app/admin.py`	The container’s start; operator commands

A route should be thin: parse, call domain code or SQL, commit, return.

Under /v1. Additive changes only within a version (new fields and routes are fine; removing or renaming isn’t): phones in the stores talk to older and newer servers.
Writes call db.commit() in the route before returning.
Optimistic concurrency: editable rows take the version the client loaded; mismatch → 409 conflict.
Idempotent creates: the client makes the id; saving the same id twice does nothing.
Never INSERT ... RETURNING or ON CONFLICT on books: both apply the read policy before the creator is a member.
Errors: return {"detail": "<code>"}. Database errors start with a code (owner_required: ...) and app/api/errors.py maps them to a status. A new code needs translations in web/src/i18n/vi.json and en.json. See Error codes.
Money: int minor units in Pydantic models. Never float.
Timezones: use the Timezone type (app/api/models.py); it maps aliases to names the database knows.
Settings: optional, with safe defaults, added to .env.example, both READMEs and Environment variables.
Admin-only routes go in routes/v1/admin.py and call a database function that checks require_admin().

Route in app/api/routes/v1/<area>.py, registered with the /v1 router.
Request/response models (ints for money, UUIDs for ids).
Tests in tests/, including another user getting 404/403 (RLS).
A paired PR to the docs (src/content/docs/ in msaving-landing): the route on API routes, and the pages it changes.
If the web app uses it: types in web/src/lib/api.ts, hooks in lib/queries.ts, in the same PR.