fix(email-themes): validate DELETE theme id as UUID

The DELETE route accepted params.id as any string, but the downstream
CUD delete handler validates it against a UUID schema. A malformed id
therefore failed inside the handler as a HexclaveAssertionError (500)
instead of a request-validation error. Constraining params.id to a UUID
keeps invalid input on the 400 path.

apps/backend/src/app/api/latest/internal/email-themes/[id]/route.tsx

@@ -62,7 +62,7 @@ export const DELETE = createSmartRouteHandler({
       tenancy: adaptSchema.defined(),
     }).defined(),
     params: yupObject({
-      id: yupString().defined(),
+      id: yupString().uuid().defined(),
     }).defined(),
   }),
   response: yupObject({