refactor(validations): split lib/validations.ts into a barrel'd folder (slice 1)

1141-line lib/validations.ts → lib/validations/<feature>.ts + primitives.ts +
an index.ts barrel. '@/lib/validations' now resolves to the barrel, so all 40
importers are untouched. Every one of the 143 exports is preserved verbatim
(verified by symbol-diff); schemas moved unchanged. tsc 0, 635 tests green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/validations/notification-prefs.ts

@@ -0,0 +1,10 @@
+import { z } from 'zod'
+import { MUTEABLE_CATEGORIES } from '../notifications-prefs-logic'
+
+// Only muteable categories are settable; billing is membership-critical and
+// never user-controllable, so it is excluded from the enum (a request to set
+// it is rejected at the boundary).
+export const notificationPreferenceSchema = z.object({
+  category: z.enum(MUTEABLE_CATEGORIES as unknown as [string, ...string[]]),
+  enabled: z.boolean(),
+})