TestImageProxyFlow in internal/server starts the database, handlers and
middleware from the constructors pixad uses, in an fx app with a fresh
state directory, and replaces only the upstream origin with a local test
server, reached through the real fetcher by its new DialContext. For a
resize with a change to JPEG and for orig, the first request answers 200
with the right content type, decoded size and X-Pixa-Cache MISS; the
second answers HIT with the same image while the upstream has had one
request; the source and the converted image are then on disk with their
rows in SQLite. TODO.md records it and drops the item from Future Steps.
Model: opus-5-5
