Brancher le backend

You are at Gate 5 of the method: connecting the backend.

Before coding, read:
- the PRD, whose first section is the strategic framing
- your frontend starter rules and conventions
- your backend conventions
- `docs/backend-requests.md`, the requests sent ahead, if there are any;
  if this flow waits on one nobody has answered yet, stop and tell me,
  and do not work around it
- the project memory and journal

Current task:
[ONE FLOW, END TO END]

Acceptance criteria:
- the user-facing flow works end to end against the real backend
- empty, loading, error and permission-denied states are handled, and so
  are the slow response, the record that already exists and the value that
  arrives malformed
- the change meets the backend conventions, checked point by point
- no privacy regression
- relevant tests are added or updated
- project memory and journal are updated if the work changes product
  behavior, architecture, stakeholder decisions, or future reasoning

Run before finishing:
- typecheck
- lint
- tests
- build
- diff check
- the checks the backend conventions name
- the slow response, the record that already exists and the malformed value,
  produced against the API itself, in tests or direct calls, since a
  browser rarely sends them
- walk the connected flow in a browser, end to end, including its empty,
  error and permission-denied states

Report which gate you are at, what is missing, what I must decide before you
cross, what changed, what was verified, the backend conventions point by
point — met, with its evidence; not applicable, with why; or not checked,
with why — any need from another team this flow revealed, written into
`docs/backend-requests.md` as a request, and any remaining product or
backend decision. Saying what you will do next is not permission to do it:
you cross when I answer, not when you announce.