Paginate album/artist/author list endpoints via parsePagination (#2840)

[atomantic]

Summary

GET / on server/routes/albums.js, artists.js, and authors.js returned the full unbounded collection, unlike notes/mood-board/loras list endpoints which use the shared parsePagination/paginateArray helpers. This adopts the same convention on all three endpoints for a uniform API contract.

• Each GET / now returns the full array by default (backward-compatible) and switches to the shared { items, total, limit, offset } envelope the moment a limit or offset query param is present (via isPaginationRequested + paginateArray, defaultLimit: 50, maxLimit: 500).
• No new helper — reuses the existing shared helpers in server/lib/validation.js.
• Follows CLAUDE.md conventions (no try/catch in handlers; helpers imported from lib/validation.js).

Test plan

• Added a "returns a bounded envelope when pagination is requested" test to each of albums.test.js, artists.test.js, and authors.test.js, asserting the items/total/limit/offset shape, plus the existing default-array tests confirm backward compatibility.
• cd server && npx vitest run routes/albums.test.js routes/artists.test.js routes/authors.test.js — 30 passed.

Closes #2840