feat(audit): stamp actor on execution + RI-exchange state transitions (#995 followup)

[cristim]

Background

The adversarial review of PR #995 (gate scheduled-purchase management on creator-scope ownership, closes #950) surfaced a pre-existing audit gap that's orthogonal to PR #995's scope but real: state-transition store methods don't record WHO performed the transition. After a Pause/Resume/Cancel/Edit action lands, the only audit trail is the row's updated_at timestamp; there's no way to attribute the action to the user who clicked.

This matters because:

1. Compliance / forensics: financial actions on cloud commitments need an actor-of-record.
2. Multi-admin orgs: with the update-any:purchases bypass introduced in sec(purchases): standard user can pause/resume other users' scheduled purchases (no ownership gate) #950, any admin can act on any purchase, but the row gives no signal of which admin did it.
3. Incident response: if a purchase is cancelled in error, today there's no quick way to find out who did it.

Scope

Affected store methods (Go, internal/config/store_postgres.go):

• TransitionExecutionStatus(ctx, executionID, fromStatuses, toStatus) — generic, used by Pause/Resume/Disable/Edit flows. Takes no actor.
• TransitionRIExchangeStatus(ctx, id, fromStatus, toStatus) — RI-exchange state machine. Takes no actor.
• TransitionRegistrationStatus(ctx, reg, fromStatus) — account-registration state. Takes no actor. (Less financially sensitive but same shape.)

Already correct (use as the reference shape):

• CancelExecutionAtomic(ctx, tx, executionID, cancelledBy *string) — accepts cancelledBy. Added recently. This is the pattern to mirror.

Proposed design

1. Schema (one migration, idempotent):

   • Add transitioned_by UUID NULL REFERENCES users(id) ON DELETE SET NULL to purchase_executions, ri_exchanges, account_registrations.
   • Add transitioned_at TIMESTAMPTZ NULL to the same tables (in case updated_at is reused for other reasons later).
   • Index transitioned_by if queries-by-actor become common (defer; add later if needed).

2. Store interface (Go):

   • Extend TransitionExecutionStatus signature with a trailing actor *string (or actor uuid.UUID) parameter. Stamps both transitioned_by and transitioned_at in the UPDATE.
   • Same for TransitionRIExchangeStatus and TransitionRegistrationStatus.
   • Mirror the cancelledBy *string nullable pattern from CancelExecutionAtomic so system-initiated transitions (scheduler tick, retry worker) can pass nil and the row is stamped with NULL (system actor).

3. Handler-side wiring:

   • Every handler call site of these methods threads the session's user ID through. Today: ~6 sites in internal/api/handler_purchases.go, ~3 in handler_ri_exchange.go, plus the scheduler tick + retry paths.
   • Scheduler tick passes nil (system-initiated). Retry paths (recover stranded approvals) pass nil. CR rate-limit timeouts pass nil.

4. Tests:

   • For each handler with the new actor param: test the row's transitioned_by matches the session user post-call.
   • For system-initiated callers: test transitioned_by IS NULL.
   • Mock surface: MockConfigStore.TransitionExecutionStatusFn already exists; extend the signature consistent with the interface change.

Out of scope (file separately if needed)

• Surfacing the actor in the History UI (UX work).
• Audit log retention policy / queryable audit endpoint.
• Stamping retroactively on existing rows (impossible without data; just NULL).
• Generalizing to other state machines (plans table, etc.) — only the three identified above are financial state machines.

Acceptance criteria

• Migration adds the columns idempotently.
• All three Transition*Status methods accept and stamp the actor.
• All handler call sites pass the session user ID.
• Scheduler / retry / system paths pass nil and stamp NULL.
• Tests assert the actor stamp for one happy path per method + one system path.
• MockConfigStore updated with the new signature; per-package mocks rely on the canonical mock (consolidated in PR feat(purchases): in-app revocation within free-cancel window (closes #290) #804).

Related

• Closes the F1 surface from the PR fix(purchases): gate scheduled-purchase management on creator-scope ownership (closes #950) #995 adversarial review report.
• Builds on PR feat(purchases): in-app revocation within free-cancel window (closes #290) #804's CancelExecutionAtomic(... cancelledBy *string) pattern.
• Builds on PR fix(purchases): gate scheduled-purchase management on creator-scope ownership (closes #950) #995's ownership gate (created_by_user_id already exists; this issue adds the per-action actor).