[danielle-tfh]

stacked on #2. per-tier download ack, split out of #2 so its easier to review

Notes

• mark_ack acks one tier and errors with SignupNotFound if theres no row - it used to be a silent no-op
• unacked_tiers lists tiers still waiting on an ack, for retry on cold start
• ran cargo test, cargo clippy --all-targets -- -D warnings, cargo fmt --check

---

Note

Low Risk
Localized vault CRUD and query changes with tests; no auth or cross-service behavior in this diff.

Overview
Adds per-tier download acknowledgment on OrbPcpStore: mark_ack sets is_download_acknowledged and updated_at for a single (signup_id, tier) and returns SignupNotFound when no row matches (no longer a silent no-op). unacked_tiers scans rows still waiting on ack so cold start can retry pending acks.

Integration tests cover tier-scoped acks and the missing-row error path.

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[danielle-tfh]

folded back into #2