Adds PrecompressedStatic handler in internal/server/static_precompressed.go,
wired by routes_static.go for /static/*filepath (bundled) and
/c/static/*filepath (custom). When a .zst or .gz sibling is on disk and
the client's Accept-Encoding allows it, the handler streams the
sibling via http.ServeContent — preserving Last-Modified and
If-Modified-Since revalidation, deriving Content-Type from the identity
filename so service-worker registration and SRI continue to work, and
setting an explicit Content-Length that http.ServeContent omits when
Content-Encoding is in effect.
Range requests always serve identity (the byte offsets in Content-Range
correspond to identity bytes), and PHOTOPRISM_HTTP_COMPRESSION=none
disables encoded-sibling selection so the operator switch stays
consistent with the runtime middleware. Vary: Accept-Encoding is
emitted only when the response could realistically vary by encoding.
Both /static/* and /c/static/* are added to NewShouldCompressFn's
exclusion list so the runtime middleware never re-encodes an
already-encoded body.
