Negotiates zstd alongside gzip via PHOTOPRISM_HTTP_COMPRESSION as a
comma-separated preference list (e.g. "zstd,gzip"); legacy "gzip",
"none", and the empty string keep behaving as before. Bypasses
Connection: Upgrade, predicate-excluded paths, HEAD (with Vary kept),
4xx/5xx, 206, 204, and 304. Weakens strong ETags on the encoding path
so caches don't reuse a compressed body for a different coding.
