feat(passkey): add storage adapters

[jmgilman]

Summary

• implement passkey.Store on memory and Postgres stores
• add Postgres passkey user/credential migration with WebAuthn credential JSONB storage
• extend shared store behavior tests to cover passkey registration, lookup, duplicate/conflict behavior, credential updates, defensive copies, and context cancellation

Validation

• go test ./passkey
• go test ./store/memory
• go test ./store/postgres
• go test ./...
• go test -tags=integration ./store/postgres
• moon run root:check --summary minimal
• git diff --check

Scope

• storage only; no testkit routes, browser JavaScript, docs, or UI/session flows