refactor into functional core/imperative shell

[swelborn]

this follows better practices in separating layers, and is easier to reason about

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This is part 8 of 10 in a stack made with GitButler:

•  10  remove jwt/jwks from this #14
•  9  add idle timeout option #13
•  8  refactor into functional core/imperative shell #12 👈
•  7  add alembic for migrations #11
•  6  use a key to dedupe reqs #10
•  5  use anyio and fixup process handling #9
•  4  systemd + vault certs #8
•  3  feat: caller dictates the cache log path; drop RUN_DIR #7
•  2  minor model updates and configuration change since timeout was added #6
•  1  create ports locally to fastcache server #5