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

Mirror the new self-service "resume after refresh" slice into the
Iteration 2 section of the curated backlog. Tracked in Gitea as #111.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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.
+
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 ## Iteration 3 — Maintenance portal and observability *(milestone: `Iteration 3 — Beheer & Observability`)*