feat(gateway): quarantine release / recipient digest UX

[schmug]

Follow-up to #32 / PR #573. Gateway quarantine holds mail in PhishSOC; there is no recipient-facing release or digest.

Task

Design + build a recipient-facing quarantine digest (periodic notification of held mail) and a self-service release flow.

Pointers

• Hold outcome: workers/lib/gateway-relay.ts (behaviorFor → hold)
• Quarantine folder: shared/folders.ts (QUARANTINE)

Decision needed

• Digest cadence and content.
• Auth model for recipient release — recipients may not have Cloudflare Access; how do they authenticate to release their own mail?

Acceptance

• Recipients receive a digest of held mail and can release a message to their backend inbox.

Out of scope

• Operator-side quarantine review (already exists).

Product/UX + auth decision required → labeled needs-decision.