fix: refuse to start when the cache directory is not writable

An unwritable cache directory degraded silently: media still served, because
the download succeeds before the cache write is attempted, so the only symptom
was one "permission denied" line per request and a cache that never filled.
Every request re-fetched from the CDN.

Probe the directory at startup and exit with the uid and the chown that fixes
it. The container image runs as uid 10000, which is the usual cause with a
bind-mounted cache, so say so in the message and in both compose examples.

0 files changed, 0 insertions, 0 deletions