feat(gateways): store inbound media as HTTP reference, not inline RDF

[retog]

What

Inbound messenger media (voice notes, images) is persisted as a durable
on-disk blob referenced by an IRI — never embedded inline in RDF as a
data: URI — regardless of size. The message record points at each blob
through a kb:attachment triple resolved over the gateway's own token-gated
GET /media/<id>.

Rationale (owner's decision): consistency over data-in-graph. One uniform
mechanism for every attachment type — small or large, image or audio — keeps the
life store pure triples, instead of a size threshold that inlines some blobs and
references others.

This also fixes the original motivation: when an inbound voice note is surfaced
into a dashboard conversation, the original audio now rides along in the
forwarded files payload, not just its (often garbled) transcript.

How

inbound_store.py

• New kb:attachment predicate (multi-valued IRI object). The media type is
  deliberately not stored in RDF — it is returned by the HTTP response's
  Content-Type when the reference is resolved, which is where a media type
  belongs once the payload lives behind a URL.
• store_media() / load_media(): blobs keyed by token_hex(16) (never an
  untrusted filename), with a <id>.type sidecar. load_media validates the id
  against a strict hex regex before touching the filesystem (path-traversal
  safe). Blobs carry no RDF extension, so qlever-dir — which indexes only
  .nt/.ttl/.n3 plus declared converters — ignores them: the binaries sit on
  the same volume as the message .nt files without ever entering the graph.
• write_message() / undelivered() carry attachment_urls (deduped, empties
  dropped).

signal / whatsapp / telegram gateways

• Read the media bytes once and persist the durable reference before
  transcription, so a failed or garbled transcript never costs the recording.
• The durable reference is stored regardless of size; only the transient,
  base64-encoded files payload forwarded to triage honours
  MAX_INBOUND_FILE_BYTES.
• Add the original voice-note audio to the forwarded files payload (size
  permitting) so the conversation carries the recording alongside its
  transcript.
• Serve GET /media/<id> (token-gated, _reply_raw).

All new steps are best-effort and non-raising: any failure forwards/persists the
message without the media link rather than dropping the message.

Tests

tests/test_inbound_image_forward.py updated for the new (files, attachment_urls) / (voice, files, attachment_urls) return contracts and the
reworded attachment note, including the oversized-image case (payload dropped,
durable reference kept). tests/test_inbound_store.py and the image-forward
suite pass; all four scripts compile.

Branch policy

Tier 3 (framework scripts/), so this goes through a PR.