docs: ADR-0004 stamdata-as-code

Document the config-as-code strategy for business-tunable reference data:
typed checked-in config validated at compile time (never a production DB),
where UI text (), reference tables (Stamdata/), and letter content
each live, and why org-templates are the deliberate runtime-editable exception.
Index it in docs/README.md and add a CLAUDE.md pointer.

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

docs/README.md

@@ -22,6 +22,7 @@ condensed, cross-linked curriculum.
 | [architecture/0001-bff-lite-decision-dtos.md](reference/architecture/0001-bff-lite-decision-dtos.md) | ADR — BFF-lite endpoints + decision DTOs (backend decides, FE renders). |
 | [architecture/0002-user-groups-and-bounded-contexts.md](reference/architecture/0002-user-groups-and-bounded-contexts.md) | ADR — user groups as actors; identity vs authorization. |
 | [architecture/0003-cibg-huisstijl.md](reference/architecture/0003-cibg-huisstijl.md) | ADR — adopt CIBG Huisstijl (vendored Bootstrap 5.2) + the token bridge. |
+| [architecture/0004-stamdata-as-code.md](reference/architecture/0004-stamdata-as-code.md) | ADR — business-tunable reference data as typed, compile-time-validated config (not a production DB). |
 | [fp-tea-atomic-design.md](reference/fp-tea-atomic-design.md) | Long-form learning guide: FP + The Elm Architecture + atomic design. |
 | [wcag-checklist.md](reference/wcag-checklist.md) | Manual WCAG checks automation can't catch (tab order, focus traps, reflow). |
 | [ui-ux-audit.md](reference/ui-ux-audit.md) | Early UI/UX audit against NL Design System (predates ADR-0003 — read in that light). |

docs/reference/architecture/0004-stamdata-as-code.md

@@ -0,0 +1,81 @@
+# ADR-0004 — Stamdata as code (config-as-code, not a production database)
+
+Status: Accepted · Date: 2026-07-20
+
+## Context
+
+The business needs to control certain inputs that change over time — the clearest example
+being **which professions link to which diplomas** (`geneeskunde → Arts`, …), but also
+tunable thresholds, policy-question text, document-category definitions, and some letter
+copy. Two hard constraints:
+
+1. **No managing this through a production database.** A live admin surface writing DB rows
+   means a bad value ships silently and is discovered in production.
+2. **Issues must be caught at compile time.** A change should be typed, reviewed, and
+   versioned before it can affect anyone.
+
+The codebase already leans this way but had never named it as a pattern, and one key table
+was neither isolated nor validated:
+
+- All reference data and thresholds are **compiled-in C# constants**, served through
+  screen-shaped BFF-lite endpoints; the frontend renders decisions and holds no reference
+  data (ADR-0001).
+- User-facing UI copy is already **`$localize`** (`src/locale/*.xlf`) — git-tracked, and a
+  second locale is a translation file, not a code change. That is already the compile-time
+  model for text.
+- The profession↔diploma map lived as a *private* `Dictionary` inside `DiplomaRules`, mixed
+  in with the rules that consume it, with **no cross-reference check**: a diploma whose
+  program wasn't in the map silently rendered `"Onbekend"`.
+
+## Decision
+
+Treat business-tunable reference data as **stamdata-as-code**: typed, checked-in
+configuration, changed through the normal git → PR → build → deploy pipeline. Never a
+production database, never runtime-editable.
+
+1. **One home, typed.** Business-editable reference data lives in the
+   `BigRegister.Stamdata` namespace (`backend/src/BigRegister.Api/Stamdata/`), one file per
+   concern, as plain typed C# data (records / dictionaries). Separate the **data** (what the
+   business tunes) from the **rules** (dev-owned logic that consumes it): the profession
+   *table* is `Stamdata.Professions`; the *rule* "an English diploma needs a B2 question"
+   stays in `DiplomaRules`.
+2. **Served unchanged.** The existing BFF-lite endpoints keep serving this data
+   (`/duo/diplomas`, `/intake/policy`, `/uploads/categories`, …). No frontend change — the
+   FE still renders decisions.
+3. **Two gates.** The **C# compiler** catches shape and type mistakes. A build-time
+   **`StamdataValidationTests`** catches the referential integrity the compiler can't —
+   every seeded diploma program resolves to a real profession, no blank keys/values,
+   thresholds in range. CI runs it, so a bad edit fails the build and never merges.
+4. **Business control = config-as-code (GitOps).** The business owns the content of these
+   files; a change is a reviewed edit, not a live DB write. A future low-code editor could
+   commit a PR on their behalf without changing this model (the compile-time gate stays).
+
+### Where each kind of business-controllable thing lives
+
+| Kind | Home | Gate |
+| --- | --- | --- |
+| Reference tables + tunable numbers (professions↔diplomas, thresholds, policy questions, document categories) | `Stamdata/` typed C# | compiler + `StamdataValidationTests` |
+| User-facing UI copy | `$localize` → `src/locale/*.xlf` | build (`i18nMissingTranslation: error`) |
+| Letter / brief passage content | config-as-code in the backend (seed content), **not** the DB | compiler + endpoint tests |
+
+### The deliberate exception: org-templates
+
+Per-organization letterhead (return address, footer, signature, margins) **is**
+runtime-editable in SQLite, via the org-template admin editor (WP-23/26). That is
+intentional and does not contradict this ADR: it is *operational configuration* owned by an
+admin persona, versioned with publish/rollback inside the app, and specific to one
+sub-organization's identity — not the shared business rules a wrong value would break for
+everyone. Stamdata (the rules and reference tables the whole register runs on) stays code.
+
+## Consequences
+
+- **+** Every change is typed, reviewed, versioned, and rollback-able through git; zero
+  production-DB risk; a dangling reference fails the build with a clear message instead of
+  reaching users.
+- **−** A change needs the PR pipeline — not instant, and a non-developer may need dev
+  assistance to edit C# (mitigated later by a low-code editor that emits a PR, or by a
+  data-file format if hand-editing ergonomics ever outweigh maximal compile-time safety).
+- **Pilot shipped with this ADR:** the profession↔diploma map extracted to
+  `Stamdata/Professions.cs`, `DiplomaRules` refactored to consume it (behaviour unchanged),
+  and `StamdataValidationTests` added. Policy-question text and document-category
+  definitions follow the same pattern as obvious next steps; not moved yet.