[bounty] Voucher clone (single-use redemption) #8

Feature

Single-use voucher redemption: exactly-once under concurrency, idempotent per (voucher, user).

The invariant / gate

Concurrent redeems of one voucher → counted once; the rest get a clean 409. Zero 5xx.

Acceptance criteria (the bar)

• Correctness gate: the concurrency/invariant probe passes on the preview — zero 5xx
• Whole test suite green from a clean checkout, including a real test for the invariant
• Deploy/observe: migrations apply; the key events are logged; health endpoints work
• No regression: existing clones' probes still pass
• Honesty: the PR description matches what actually runs; AI use disclosed

Multiple people may claim this — the strongest implementation gets merged; everyone gets feedback. Comment "claiming" to take it.