docs: close TECH-AXIOS-STORE-COUPLING and add TECH-AXIOS-INTERCEPTOR-TESTS

Removes the closed TECH-AXIOS-STORE-COUPLING entry from BACKLOG.md
(the structural decoupling landed in 53f6a7b + 26a92b3). The
git-history search `git log --grep=TECH-AXIOS-STORE-COUPLING`
remains the durable closure record, per the backlog hygiene
convention.

Adds a follow-up entry TECH-AXIOS-INTERCEPTOR-TESTS that captures
all four acceptance scenarios (X-Organisation-Id header
injection, 401 auth-fail flow, 403+impersonation_ended revocation
flow, 4xx/5xx error toast). Phase A audit found that none of
these is tested today; the refactor is gedragsneutraal so no
regression was introduced, but the gap is real and should not
silently outlive the refactor that made it visible. Priority
medium per Bert's Phase B sign-off.

Appends the debt-closed sentence to the Sessie 1c entry in
ARCH-CONSOLIDATION-2026-04.md, citing commit 53f6a7b.

Co-Authored-By: Claude <noreply@anthropic.com>

dev-docs/ARCH-CONSOLIDATION-2026-04.md

@@ -574,7 +574,11 @@ Zie §4 voor scope en stappen.
   enforcement na PR-B), `TECH-WS3-BOUNDARIES-ROUTER-ZONE` (matrix-update
   wanneer `plugins/1.router/` naar `router/` verhuist). WS-3 lint cleanup
   + boundaries enforcement effectief afgerond; volgende WS-3 stap is PR-B
-  (portal merge) zodra WS-6 sessie 2 in main is geland.
+  (portal merge) zodra WS-6 sessie 2 in main is geland. Debt closed in
+  commit `53f6a7b`: `lib/axios.ts` decoupled from stores via
+  `registerInterceptors(client, deps)` callback seam,
+  `plugins/3.axios-bindings.ts` provides the runtime wiring; the four
+  per-line disables are removed.
 
 **Klaar-criteria:**
 - `apps/portal/` is verwijderd