feat(backend): expand stamdata + OpenZaak-ready cases seam (WP-49)

Stamdata: add beroepen, opleidingen (temporal), and specialismen tables to the
schema-driven catalog (zero UI code). opleidingen.beroep and specialismen.beroep
both reference beroepen.code — the first stamdata->stamdata references, enforced by
two new StamdataRef entries in the CI gate.

OpenZaak/ZGW (WP-49, slice 1 — read-only zaken): introduce IZaakSource as the cases
read seam. Default LocalZaakSource reads the local SQLite store (offline); an
OpenZaakZaakSource (Zgw/ client: HS256 per-call JWT, ZGW->existing-DTO mapper,
paginating HTTP source) is selected behind Zgw:Enabled (default false). The FE never
changes — same ApplicationSummaryDto, no api-client drift. Unit-tested with fixtures
+ a stub HttpMessageHandler; no live OpenZaak needed.

Docs: ADR-0005, reference/openzaak-integration.md, WP-49..52 roadmap, stamdata.md
update, README index rows.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/reference/stamdata.md

@@ -6,6 +6,11 @@ build**, not a runtime-editable database. For the _why_, see
 [ADR-0004 — Stamdata as code](architecture/0004-stamdata-as-code.md); this page is _how the
 code is laid out and how to add a table without coupling_. Built in WP-29, hardened in WP-48.
 
+Tables today: `professions` (opleiding-program → beroep), `beroepen` (the BIG professions master
+list), `opleidingen` (temporal; `beroep` → `beroepen.code`) and `specialismen` (`beroep` →
+`beroepen.code`). The last two are **stamdata → stamdata** references — one table keyed on by two
+others — enforced by the CI gate below.
+
 ## The one rule that shapes everything: no runtime write path
 
 The catalog is the source of truth and lives in code. The admin editor **downloads** an
@@ -50,8 +55,11 @@ component**. This is the payoff of the schema-driven design.
 
 `backend/tests/BigRegister.Tests/StamdataValidationTests.cs`. `Every_catalog_table_is_valid`
 covers every registered table generically; the `StamdataRef` list catches dangling
-references (today: `Diploma.Opleiding → professions.program`). A bad edit, an orphaning
-delete, or a premature expire **fails the PR build** — never prod.
+references — both seed → stamdata (`Diploma.Opleiding → professions.program`) and
+stamdata → stamdata (`Opleiding.beroep → beroepen.code`, `Specialisme.beroep →
+beroepen.code`). A bad edit, an orphaning delete, or a premature expire **fails the PR
+build** — never prod. Adding a cross-table FK is one `StamdataRef` entry: the referencing
+keys + a resolver against the target table's (valid-today) keys.
 
 ## Coupling