books: import zod as a namespace so tests pass under the Bun runtime in CI

The Docker build runs vitest via bun (no Node in oven/bun), where the named
`{ z }` import from zod 4 resolved to undefined; the rest of the repo already
uses `import * as z from 'zod'`.

src/lib/books/progress.ts

@@ -1,5 +1,5 @@
 // Validation and maths for the reader's progress API (pure, unit tested).
-import { z } from 'zod';
+import * as z from 'zod';
 
 export const progressPayload = z.object({
 	fileId: z.number().int().positive().nullish(),

src/lib/server/books/notes.ts

@@ -1,6 +1,6 @@
 // Highlights and notes (books.notes). A row with `highlight` is a highlight from the reader
 // (optionally with a note); a row with only `note` is a free-form note on the book.
-import { z } from 'zod';
+import * as z from 'zod';
 import { and, eq } from 'drizzle-orm';
 import { db } from '#lib/server/db/index.js';
 import { bookFiles, bookNotes, bookWorks } from '#lib/server/db/schema/index.js';