Add CSV bulk-import for authors/books

## What problem does this solve?

Seeding a real catalog today means one `POST /api/v1/authors` and one `POST /api/v1/books` call per row — fine for the Postman/Bruno demo data, painful for anyone migrating an existing library's spreadsheet or another system's export into this one.

## Proposed solution

- Add `POST /api/v1/books/import` and `POST /api/v1/authors/import` (JWT-gated, `multipart/form-data` CSV upload) that parse rows, validate each with the existing `go-playground/validator` DTOs, and insert them in a single transaction — returning a per-row success/error report (so one bad row doesn't silently drop the rest, and the caller can see exactly which rows failed and why).
- Reuse the existing author-lookup-or-create-by-name logic if a book row references an author name that doesn't exist yet, to keep imports self-contained.
- Add a `books import ` CLI subcommand that streams the file to the endpoint and prints the row report.

## Alternatives considered

A one-off script outside the API — works for a single migration but isn't reusable, isn't testable the way an endpoint is, and doesn't benefit future users who also need to bulk-load data.

## Area

Books, Authors

## Priority

Nice to have — good first issue, since it's mostly plumbing on top of validation/insert logic that already exists per-row.