Adds attachment sharing over the mesh: a ContentRef envelope (variant 19)
carries the blob CID, size, mime, optional thumb/caption, and a per-peer
HMAC capability URL so the recipient fetches the full blob out-of-band via
`GET {sender_onion}/blob/{cid}?cap=..&exp=..&peer=..`. BlobStore is shared
from ApiHandler into RpcHandler so mesh.send-content and mesh.fetch-content
(reqwest via TOR_SOCKS_PROXY) hit the same store and cap_key.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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