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>

CLAUDE.md

@@ -117,6 +117,12 @@ touches only `infrastructure/` + `contracts/` (see ARCHITECTURE §6). Server-own
 rules stay in `domain/*.policy.ts` as reference impl + unit test, marked server-owned,
 but the FE doesn't call them.
 
+**Business-tunable reference data ("stamdata") is config-as-code, not a DB.** Tables the
+business controls (profession↔diploma map, thresholds, policy-question text) live as typed
+C# in `backend/.../Stamdata/`, validated at build by `StamdataValidationTests` (a bad edit
+fails CI, never prod) — never runtime-editable. Org-templates are the deliberate exception
+(operational per-org config in SQLite). UI copy is `$localize`. See ADR-0004.
+
 ### 5. Testing
 
 Vitest. Co-locate `*.spec.ts` next to the unit. **Domain and pure logic must have a