PlaceholderFor had no callers at all. Its comment offered it to callers that
must not consult persisted state, but no such caller was ever written, so it
is removed rather than unexported.
entityExists and encode83 are only reached from inside their own packages;
their tests are package-internal (artwork) or never touch them (blurhash_test).
ImageStore stays exported: server/subsonic/e2e constructs one to wire up the
worker, and that suite cannot move into package artwork.
Signed-off-by: Deluan <deluan@navidrome.org>
3ce92cc055