Issue 009: Event Sourcing for Workflow State

Summary

Replace the in-place mutations of workflow_tasks with an
append-only workflow_events log; the existing row becomes the
latest projection. Adds a snapshot policy so replay is bounded,
and a ./scripts/replay-workflow.sh CLI for post-incident
debugging.

Motivation

• ROADMAP.md lists "Persist workflow transition history for
  audit and replay" as Near Term.
• The current mutable row loses information about why a
  transition happened (the lease owner, the recovery scheduler's
  reasoning, the prior stage, etc.).
• Event sourcing is a one-shot investment that unlocks replay, audit
  trails, and future analytics projections.

Tasks

• V17 migration: workflow_events, workflow_snapshots,
  per-task seq index, type-time index for ad-hoc queries.
• eventsourcing/workflow/WorkflowEvent.java interface and
  nine concrete event records
  (Created, LeaseAcquired, LeaseRenewed, StageEntered,
  Succeeded, Failed, Retried, Recovered,
  DeadLettered).
• eventsourcing/workflow/WorkflowEventStore.java
  (append, loadSnapshot, replayFrom).
• eventsourcing/workflow/WorkflowTask.applyEvent(...) as a
  pure-function update on the record.
• eventsourcing/workflow/WorkflowTaskProjector.java
  transactional consumer that materialises projections.
• Refactor WorkflowOrchestrator to emit events instead of
  mutating workflow_tasks directly.
• Refactor WorkflowRecoveryScheduler similarly.
• eventsourcing/workflow/WorkflowSnapshotService.java with
  the policy: 100 events / 5 minutes / any pending change.
• scripts/replay-workflow.sh: --since=, --task-id=,
  --dry-run.
• docs/operations.md: replay recipes, snapshot retention.
• Testcontainers ITs (from RFC 001): rewrite the workflow
  cases to drive the event log; add 5 dedicated event-sourcing
  ITs.

Acceptance criteria

• A workflow task exercising the entire happy path now produces
  ~10 rows in workflow_events plus an updated
  workflow_tasks projection.
• After process restart mid-task, the projection recovers exactly
  to the prior state by replaying events from the latest
  snapshot.
• ./scripts/replay-workflow.sh --since=2026-08-01T00:00:00Z --task-id=$TASK_ID --dry-run prints the full event sequence
  for that task.
• Existing /actuator/workflow metrics continue to work; new
  finsight.workflow.event MeterRegistry timer is exposed.

Out of scope

• Cross-bounded-context events.
• Read-model projections beyond workflow_tasks.
• Full event-schema upcasting across major versions.

References

• docs/rfcs/RFC-009-event-sourcing-workflow-state.md
• ROADMAP.md (Near Term: persist workflow transition history)
• backend/src/main/java/com/finsight/workflow/WorkflowOrchestrator.java
  (target of refactor)
• backend/src/main/java/com/finsight/workflow/WorkflowRecoveryScheduler.java
  (target of refactor)

Estimate

8 weeks. Split into 7 PRs:

1. V17 migration + event store + 9 events (≈ 800 LoC, 1 PR)
2. WorkflowTask.applyEvent + projector (≈ 600 LoC, 1 PR)
3. WorkflowOrchestrator refactor (≈ 500 LoC, 1 PR)
4. WorkflowRecoveryScheduler refactor (≈ 300 LoC, 1 PR)
5. WorkflowSnapshotService + policy (≈ 400 LoC, 1 PR)
6. Replay tool + docs (≈ 400 LoC, 1 PR)
7. Workflow ITs rewritten + 5 new event-sourcing ITs (≈ 700
   LoC, 1 PR)