Conditional GET cache for periodic data sources. Persists ETag and
Last-Modified to a .meta sidecar so 304s survive process restarts.
Two independent rate-limit gates (MaxAge file-mtime, MinInterval
in-memory). Body cap via MaxBytes defends against fill-disk from a
hostile upstream or redirect target. Authorization (and any
caller-supplied) headers stripped on every redirect hop. Userinfo
in URL is redacted from error messages.
