gleam: implement request handlers and router

[HeyItWorked]

Task

Create gleam-bookshelf/src/handlers.gleam and gleam-bookshelf/src/router.gleam.

Router pattern

Gleam uses pattern matching on method + path segments instead of registering routes:
```gleam
case req.method, wisp.path_segments(req) {
Post, ["books"] -> create_book(req, ctx)
Get, ["books"] -> get_books(ctx)
Get, ["books", id] -> get_book(id, ctx)
Put, ["books", id] -> update_book(req, id, ctx)
Delete, ["books", id] -> delete_book(id, ctx)
_, _ -> wisp.not_found()
}
```

Handlers to implement

Handler	Input	DB call	Response
create_book	JSON body	insert_book	201 + book JSON
get_books	nothing	get_all_books	200 + list JSON
get_book	id from path	get_book_by_id	200 + book JSON
update_book	id + JSON body	update_book	200 + book JSON
delete_book	id from path	delete_book	204 no content

Key differences from Go/TS

• JSON decoding requires building a decoder (like Go's Scan, but for JSON)
• JSON encoding requires an explicit encode_book(book) -> Json function
• No early returns — use case expressions or use for control flow
• Path param parsing — int.parse(id_string) returns Result(Int, Nil)

Acceptance criteria

• All 5 handlers implemented
• Router pattern-matches all methods + fallback to 404
• JSON decode validates title + author required, status defaults to "want to read"
• Invalid status returns 400
• gleam build passes