docs(backlog): add S-26 self-service resume slice (refs #111) (#112)

## What & why

Mirror the new self-service **"resume after refresh"** slice into the Iteration 2 section of the curated backlog (`BACKLOG.md`), keeping it in sync with Gitea. Tracked as #111 (S-26).

Refs #111 — **does not close it**: the backlog is the curated mirror, the slice itself stays open for implementation.

## Definition of Done

- [x] Linked Gitea issue (#111).
- [ ] Failing test committed before the implementation — N/A (docs-only backlog mirror).
- [ ] Implementation makes the test pass; refactor commit if structure improved — N/A.
- [x] Conventional Commits referencing the issue (`refs #111`).
- [ ] CI green — no code paths touched; only `BACKLOG.md`.
- [ ] `docker compose up` reaches green health checks — N/A.
- [x] Docs updated (this IS the docs change).
- [ ] ADR added — N/A.
- [ ] Demo note in `docs/demo-script.md` — N/A (backlog entry, not a shipped user-visible change).

## Notes for reviewers

Single-file change: adds the `S-26` entry (Outcome + Acceptance) after S-14 in Iteration 2, matching the surrounding slice format. The `S-B04` local-stack bug (#110) is intentionally **not** added — the `S-B0N` bug-slices have never been mirrored in `BACKLOG.md` (they live only in Gitea).

Reviewed-on: #112

BACKLOG.md

@@ -239,6 +239,12 @@ Split (issue #11 closed) into two independently-demoable slices per §13 — the
 
 **Outcome:** Boundary timer on beoordeling user task — 14 days. On timeout, reassigns to a teamlead role.
 
+### S-26 · Self-service — resume an existing registration after refresh — #111
+
+**Outcome:** a signed-in zorgprofessional who reloads the self-service portal (or returns later) gets back to their in-flight registration and its actions (Documenten aanleveren, Trek aanvraag in), instead of a blank submit form with the reference lost. Today all post-submit state lives in in-memory signals, the reference is not in the URL, and there is no self-service read endpoint — so a reload strands the registration. Adds an owner-scoped (DigiD bsn) `GET /self-service/registrations` on the BFF/domain and a load-on-init/route restore in the portal.
+
+**Acceptance:** BDD — resume after refresh shows the existing registration; lookup is owner-scoped (never another citizen's); a user with no in-flight registration still sees the submit form. Playwright e2e reloads mid-flow and asserts the actions remain reachable.
+
 ---
 
 ## Iteration 3 — Maintenance portal and observability *(milestone: `Iteration 3 — Beheer & Observability`)*