feat(stamdata): admin stamdata maintenance editor (beheer)

Realizes ADR-0004's "future low-code editor that commits a PR": an
admin-only stamdata maintenance editor built on the stamdata-as-code
foundation.

Backend: `professions` moves from a hardcoded C# dictionary to an embedded
`professions.json` data-file (typed as `ProfessionMapping`) with valid-time
(geldigVan/geldigTot, half-open). A generic, reflection-driven
StamdataCatalog/StamdataTable/StamdataFile describes every table so one
endpoint pair + one grid editor serve all of them; add a table in one line.
Two read-only, admin-gated endpoints (GET /stamdata, GET /stamdata/{table}
?peildatum=) — no runtime write path. Generic build gate
`Every_catalog_table_is_valid` (keys non-blank, no overlapping validity,
well-formed windows).

Frontend: new `beheer` context (route beheer/stamdata, capabilityGuard
'stamdata:edit'). A schema-driven grid editor edits rows locally; download()
emits {table}.json for the admin to commit as a reviewed PR (no mutation
command — the CI build + StamdataValidationTests stay the authority).

Full gate GREEN both sides; gen:api leaves no drift; new stamdata story
passes axe. See WP-29 + ADR-0004.

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

backend/src/BigRegister.Api/Contracts/Dtos.cs

@@ -46,6 +46,15 @@ public sealed record DuoLookupDto(IReadOnlyList<DuoDiplomaDto> Diplomas, ManualD
 
 public sealed record IntakePolicyDto(int ScholingThreshold);
 
+// --- Stamdata maintenance (ADR-0004): generic, schema-driven so ONE contract serves
+// every business-editable table. Columns are reflected from the table's typed record;
+// Rows are the raw JSON objects (opaque here — the editor renders them by column type).
+public sealed record StamdataColumnDto(string Name, string Type, bool IsKey, IReadOnlyList<string>? Options);
+public sealed record StamdataTableSummaryDto(string Id, string Label, IReadOnlyList<StamdataColumnDto> Columns, bool Temporal);
+public sealed record StamdataTableDto(
+    string Id, string Label, IReadOnlyList<StamdataColumnDto> Columns, bool Temporal,
+    IReadOnlyList<System.Text.Json.JsonElement> Rows);
+
 // --- Document upload contracts ---
 
 public sealed record DocumentCategoryDto(