refactor: extract API handlers into internal packages

Move HTTP handler logic out of main into internal api and server packages,
slimming main to a thin entrypoint. Bundles bug fixes: login nil-panic,
empty-password validation, chirp sort ordering, duplicate-email 409, and
restricting the file server to the assets directory.

sql/queries/users.sql

@@ -31,5 +31,3 @@ UPDATE users
 SET is_chirpy_red = TRUE
 WHERE id = $1
 RETURNING *;
-
-