feat: lifecycle events carry triggered_by_assistant_turn_id back-reference (T114.1)

Phase 3.5 T83.4 surfaced un-rolled-back lifecycle transitions on
regenerate; T114 wires up the actual rollback. Step 1 is the back-
reference: every event_started / event_completed / event_cancelled
emitted by post_turn (chat/web/turns.py) and regenerate
(chat/services/regenerate.py) now carries
``triggered_by_assistant_turn_id`` in its payload, set to the id of
the assistant_turn event that produced the transition.

Schema decision (Option A from the plan): no migration. The field is
a payload convention only — older event_log rows lack it and rollback
will skip them with a debug log when T114.3 lands. Forward-only.

The post_turn lifecycle block already runs AFTER the assistant_turn
event is appended (step 8a vs step 7), so ``primary_assistant_event_id``
is in scope. Same for regenerate: the lifecycle classification (step 9a)
runs after step 6's append. **No emission-order reorder was needed**
in either flow.

Updates ``test_turn_with_event_transition_appends_started_event`` to
assert the new field is present in the emitted event_started payload
and points at the assistant_turn id.

tests/test_turn_flow.py

@@ -1023,6 +1023,18 @@ def test_turn_with_event_transition_appends_started_event(
         assert started_payload["event_id"] == "evt_1"
         assert started_payload["started_at"] == "2026-04-26T20:00:00+00:00"
 
+        # T114.1: payload carries the back-reference to the assistant_turn
+        # that triggered the transition. The assistant_turn lands in
+        # event_log immediately before the event_started, so its id is
+        # the largest assistant_turn id in the chat at this point.
+        at_id = conn.execute(
+            "SELECT id FROM event_log "
+            "WHERE kind = 'assistant_turn' "
+            "  AND json_extract(payload_json, '$.chat_id') = 'chat_bot_a' "
+            "ORDER BY id DESC LIMIT 1"
+        ).fetchone()[0]
+        assert started_payload["triggered_by_assistant_turn_id"] == at_id
+
         # The events projection row reflects the active status.
         ev_row = conn.execute(
             "SELECT status, started_at FROM events WHERE event_id = ?",