docs(backlog): add WP-18..22 (productie-volwassenheid phase)

Gap analysis found the POC's designed-but-unbuilt strategic gaps: ABAC
authorization (ADR-0002/PRD-0002 phase P1), no e2e coverage, unproven
i18n second-locale seam, thin resilience seams (correlation-id,
idempotency, retry), and in-memory-only persistence. Each WP is grounded
in the current code (file paths + line numbers), not just the analysis.

Also corrects PRD-0001's stale 'Proposed' status header — the Mijn
aanvragen vertical is fully built.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docs/prd/0001-mijn-aanvragen-en-wizardstatus.md

@@ -1,6 +1,6 @@
 # PRD 0001 — "Mijn aanvragen": running wizards, application status & document preview
 
-Status: Proposed · Date: 2026-07-01 · Context: SSP / Zorgverlener (see ADR-0002)
+Status: Implemented · Date: 2026-07-01 · Context: SSP / Zorgverlener (see ADR-0002)
 
 > Cross-references: **ADR-0001** (BFF-lite endpoints + decision DTOs) and **ADR-0002** (user groups as
 > actors; the `Concept → In behandeling → Goedgekeurd/Afgewezen` aanvraag lifecycle). This PRD