fix(structures): widen via String() in narrowing test to satisfy lint

The narrowing test for `isStructureKind` had `const candidate: string =
'medico_social'`, which ESLint's `@typescript-eslint/no-inferrable-types`
correctly flags as trivially-inferred.

The naive fix - dropping the annotation - would let TypeScript infer
the literal type `'medico_social'`, which is already a StructureKind
subtype. The narrowing check would then be vacuous (TypeScript already
knows the value is a kind, the runtime guard has nothing to prove).

Fix: round-trip through `String()` to widen the literal to `string`.
TypeScript infers `string` for `String(...)` return; no annotation
needed; the narrowing test stays meaningful.

CI lint target on portal-bff now passes 0 errors. The 13 remaining
warnings (non-null assertions in principal-extractor.spec, unused
underscored params in rate-limit.middleware, etc.) are pre-existing
and not in scope for this fix.

apps/portal-bff/src/structures/structure-kind.spec.ts

@@ -35,7 +35,10 @@ describe('isStructureKind', () => {
   });
 
   it('narrows the type at the call site', () => {
-    const candidate: string = 'medico_social';
+    // Round-trip through `String()` to widen the literal to `string`.
+    // Without this the inferred type is `'medico_social'`, already a
+    // StructureKind subtype — the narrowing check would be vacuous.
+    const candidate = String('medico_social');
     if (!isStructureKind(candidate)) {
       throw new Error('unreachable — candidate is a known kind');
     }