Persist immutable audit events for every state transition
epic-expense-approval-workflow-core-logic-task-006 — After each successful state transition in ApprovalWorkflowService, write an immutable ClaimEvent record via ClaimEventsRepository. Each event must capture: claim ID, from-state, to-state, actor user ID, actor role, UTC timestamp, and optional coordinator comment. Events must never be mutated or deleted after creation.
Acceptance Criteria
Technical Requirements
Execution Context
Tier 4 - 323 tasks
Can start after Tier 3 completes
Implementation Notes
Use Supabase's `.insert()` on the claim_events table immediately after the status update `.update()` call. To achieve transactionality without a Supabase Edge Function, wrap both operations in a Supabase RPC (PostgreSQL function) that performs both writes in a single database transaction. This is the safest approach — define a `record_claim_transition(p_claim_id, p_from_status, p_to_status, p_actor_id, p_actor_role, p_comment)` SQL function and call it via `supabase.rpc()`. The RLS on claim_events should be configured as: ENABLE ROW LEVEL SECURITY, then only a single INSERT policy with `USING (organisation_id = auth.jwt() ->> 'organisation_id')`.
Never add an UPDATE policy. The ClaimEventsRepository should expose only `insertEvent()` and `getEventsForClaim()` methods — no update or delete methods should exist on the repository class.
Testing Requirements
Unit tests with flutter_test and mocked ClaimEventsRepository. Test cases: (1) event written with correct from/to states for each of the 4 transitions, (2) event not written if state transition repository call fails (rollback behaviour), (3) actor_id and actor_role correctly populated from auth context, (4) optional comment stored when provided and null when absent, (5) comment sanitisation strips unsafe characters, (6) duplicate event write attempt is handled gracefully (idempotency). Integration test: verify RLS blocks UPDATE and DELETE on real Supabase test environment.
The ThresholdEvaluationService is described as shared Dart logic used both client-side and in the Edge Function. Supabase Edge Functions run Deno/TypeScript, not Dart, meaning the threshold logic must be maintained in two languages and can diverge, causing the server to reject legitimate client submissions.
Mitigation & Contingency
Mitigation: Implement the threshold logic as a single TypeScript module in the Edge Function and call it via a thin Dart HTTP client wrapper for client-side preview feedback only. The server is always authoritative; the client version is purely for UX (showing the user whether their claim will auto-approve before they submit).
Contingency: If dual-language maintenance is unavoidable, create a shared golden test file (JSON fixtures with inputs and expected outputs) that is run against both implementations in CI to detect divergence immediately.
A peer mentor could double-tap the submit button or a network retry could trigger a duplicate submission, causing the ApprovalWorkflowService to attempt two concurrent state transitions from draft→submitted for the same claim, potentially resulting in two audit events or conflicting statuses.
Mitigation & Contingency
Mitigation: Implement idempotency in the ApprovalWorkflowService using a database-level unique constraint on (claim_id, from_status, to_status) per transition, combined with a UI-level submission lock (disable button after first tap until response returns).
Contingency: Add a deduplication check at the start of every state transition method that returns the existing state if an identical transition is already in progress or completed within the last 10 seconds.
Claims with multiple expense lines (e.g., mileage + parking) must have their combined total evaluated against the threshold. If individual lines are added asynchronously or the evaluation runs before all lines are persisted, the auto-approval decision may be computed on an incomplete set of expense lines.
Mitigation & Contingency
Mitigation: The Edge Function always fetches all expense lines from the database (not from the client payload) before computing the threshold decision. Define a clear claim submission contract that requires all expense lines to be persisted before the submit action is called.
Contingency: Add a validation step in ApprovalWorkflowService that counts expected vs. persisted expense lines before allowing the transition, returning a validation error if lines are missing.