Publishers no longer need to self-host. When the service is configured
with PROMETHEUS_S3_* credentials pointing at the hosted s3d
(s3.silentmode.st), every clean upload is pushed to
s3://${bucket}/${prefix}<sha256>.bin and the public URL comes back on
/api/upload as `hosted_url`. Step 2 of the submit form pre-fills the
Download URL input with that URL — publishers can overwrite if they
prefer their own hosting; by default they ship nothing.
- service/s3.js: SigV4 S3 client vendored from Pithos/core/s3.js so bug
fixes port by file-copy. Path-style, UNSIGNED-PAYLOAD streaming PUT,
Content-Length required (s3d chunked encoding does not round-trip
cleanly for large bodies).
- service/storage.js: thin wrapper exposing isConfigured() and
uploadScannedFile(). Idempotent on sha256 — same bytes → same key.
Failure is non-fatal; the publisher can still paste their own URL.
- service/uploads.js: after rename-to-final, push to Sia if configured.
Added `hosted_url` to the response (null when storage off).
- service/pages.js: step 2 pre-fills download_url when hosted_url is
present; hint text explains the override path.
- docker-compose.yml: PROMETHEUS_S3_* env plumbed through, with sane
defaults for bucket/prefix (bns + prometheus-storage/).
