v0.5.2 — bytes_served_total counted HEAD requests

bytes_served_total counted HEAD requests. A HEAD carries the full
content-length and transfers no body; huggingface_hub HEADs every file in a
repo before downloading any of it. Measured: 5 HEADs of a 10,985-byte file
added exactly 54,925. Every historical served figure from this cache is
inflated by the metadata traffic that preceded the transfers. Ingested was
always correct.

HEALTHCHECK now exercises /metrics, not only /healthz.

Documents three limits found in production: the 46.57 GiB plain-HTTP ceiling
(decimal, not the 50 the constant's name implies), HF_HUB_DOWNLOAD_TIMEOUT's
10s default timing out the first node to want a model, and that only /healthz
is a health endpoint.