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>

backend/tests/BigRegister.Tests/StamdataValidationTests.cs

@@ -18,12 +18,26 @@ public class StamdataValidationTests
   /// which steers the editor toward closing validity only once nothing current relies on it.
   private sealed record StamdataRef(string Description, IEnumerable<string> Keys, Func<string, bool> Resolves);
 
+  // The beroepen master-list keys every other profession table points at.
+  private static readonly IReadOnlySet<string> BeroepCodes =
+      StamdataFile.Load<Beroep>("beroepen").Select(b => b.Code).ToHashSet(StringComparer.Ordinal);
+
   private static readonly IReadOnlyList<StamdataRef> References = new[]
   {
     new StamdataRef(
         "Diploma.Opleiding → professions.program (valid today)",
         SeedData.Diplomas.Select(d => d.Opleiding),
         key => Professions.ByProgram.ContainsKey(key)),
+    // Stamdata → stamdata references: two tables point at beroepen.code, so deleting or
+    // renaming a beroep that either still uses fails the build (WP-48 gate, generalized).
+    new StamdataRef(
+        "Opleiding.beroep → beroepen.code",
+        StamdataFile.Load<Opleiding>("opleidingen").Select(o => o.Beroep),
+        key => BeroepCodes.Contains(key)),
+    new StamdataRef(
+        "Specialisme.beroep → beroepen.code",
+        StamdataFile.Load<Specialisme>("specialismen").Select(s => s.Beroep),
+        key => BeroepCodes.Contains(key)),
   };
 
   [Fact]