gleam db: get_all_books and get_book_by_id (Read)

[HeyItWorked]

Sub-issue of #9, depends on #9a (shared helpers)

pub fn get_all_books(conn) -> Result(List(Book), AppError)
pub fn get_book_by_id(conn, id) -> Result(Book, AppError)

• get_all_books: SELECT id, title, author, status FROM books — return returned.rows directly
• get_book_by_id: same query with WHERE id = $1 — empty rows → Error(NotFound)