gleam db: AppError type, book_decoder, and map_error helper

[HeyItWorked]

Sub-issue of #9

Scaffold db.gleam with shared infrastructure before the CRUD functions:

• AppError custom type — NotFound | DatabaseError(String)
• book_decoder() — decodes (id, title, author, status) row tuple into a Book using gleam/dynamic/decode, converts status string via status_from_string
• map_error() — converts pog.QueryError → AppError via string.inspect

Imports: gleam/dynamic/decode, gleam/result, gleam/string, models, pog

[HeyItWorked]

What and why

AppError type — pog returns its own error types (QueryError). Our handlers don't care about pog internals, they care about two things: "not found" or "something broke." So we wrap pog's errors into our own simple type.

book_decoder() — Postgres returns rows as untyped tuples. Gleam won't let you use untyped data. The decoder is the recipe: "column 0 is an int, column 1 is a string..." etc. Without it, pog has no idea how to turn a row into a Book. Go does this with Scan, TS gets it for free — Gleam makes you be explicit.

map_error() — One-liner glue. Every CRUD function will call pog.execute which returns Result(_, pog.QueryError). Our functions return Result(_, AppError). This helper converts between the two so we don't repeat the conversion five times.

Three things, all plumbing so the CRUD functions (#18–#21) stay clean.