feat(poll): enforce auditable lifecycle transitions

README.md

@@ -63,6 +63,30 @@ Poll stores the shared poll/options/responses/result summary. Scheduling owns
 the domain workflow around rooms, calendars, free/busy checks, reminders,
 appointment creation, and optional Workflow integration.
 
+## Poll lifecycle
+
+Poll configures its lifecycle through the separate, pure transition engine in
+`backend/transitions.py`. The default policy is:
+
+- `draft` → `open` or `archived`
+- `open` → `draft`, `closed`, or `archived`
+- `closed` → `open`, `decided`, or `archived`
+- `decided` → `open`, `decided` (an explicit re-decision), or `archived`
+- `archived` → the status from which the Poll was archived
+
+Reopening and archiving preserve responses, close history, and decision
+metadata. Every applied transition has a Poll-owned lifecycle audit record.
+Re-deciding always appends a record and supersedes the current decision. An
+exact retry carrying the same `Idempotency-Key` is a no-op and returns the
+original transition record; reusing that key for a different action or option
+is rejected. Without an idempotency identity, a repeated transition is invalid
+except for the deliberately auditable `decided` → `decided` action.
+
+Poll API representations expose every lifecycle action with its availability
+and, when unavailable, the policy reason. Management clients can use the
+`GET /poll/polls/{poll_id}/transitions` endpoint to read the durable history
+instead of duplicating the matrix in their UI.
+
 ## Development Install
 
 ```bash