fix(intake): name the missing recordings in the same refusal

[haribo]

Summary

• The frugal contract promises one refusal naming every missing address, but checkCaptures built the missing-content list from the captures alone: a client pushing captures and videos got two 409s instead of one (name the missing recordings in the same refusal as the captures #223, reproduced against a local server)
• Absent recordings now join the same missing-content answer — existence checked only, never their format (ADR 0013)
• The intake Blobs port gains Exists, already carried by the file store

Test plan

• TestAMissingRecordingIsNamedInTheFirstRefusal — written first, failed with the exact observed symptom (the refusal named the capture alone), passes now
• Full gates: gen-check, be-check, be-test, db-test, fe-test-e2e (62)

Test helper extended, no assertion changed: refusingBlobs implements the new Exists with the same refuse-first guarantee.

Closes #223