gleam: implement DB layer with gleam_pgo

[HeyItWorked]

Task

Create gleam-bookshelf/src/db.gleam with all 5 CRUD database functions.

Functions to implement

• insert_book(conn, title, author, status) -> Result(Book, AppError)
• get_all_books(conn) -> Result(List(Book), AppError)
• get_book_by_id(conn, id) -> Result(Book, AppError)
• update_book(conn, id, title, author, status) -> Result(Book, AppError)
• delete_book(conn, id) -> Result(Nil, AppError)

Key differences from Go/TS

• No global mutable state — conn (pgo.Connection) is passed as an argument, not stored in a module-level variable
• Row decoder — must build a dynamic.decode4(...) decoder to map SQL columns to Book fields
• use keyword — replaces Go's if err != nil pattern for chaining fallible operations
• SQL is identical — same $1, $2 parameterized queries, same RETURNING clauses

Acceptance criteria

• All 5 functions implemented with proper Result return types
• Row decoder correctly maps (id, title, author, status) columns
• delete_book returns Error(NotFound) when 0 rows affected
• gleam build passes