Introduce a static config map
Introduce a static config map to pass around - avoids runtime getenv calls, makes a bunch of stuff pure again, and makes for smaller function signatures.
The idea is that this map is constructed fomr environment vars at start-up and is never changed or updated.
The map has four fields, and generally looks like this:
{:path (or (System/getenv "LYDBOG_PATH") "/tmp/")
:public-url (or (System/getenv "PUBLIC_URL") "http://localhost:3000/")
:podcast-name (or (System/getenv "PODCAST_NAME") "Lydbog")
:art-url (or (System/getenv "ART_URL") "http://localhost:3000/art.png")}