fix: publish schedules after creation watermark

src/wf_api/schedules.py

@@ -21,7 +21,7 @@ from wf_scheduling.history import FileScheduleHistoryRecorder, HistoryEntry
 from wf_scheduling.models import OccurrenceRecord, Schedule
 from wf_scheduling.occurrences import occurrence_id
 from wf_scheduling.prepare import PreparationRejected, SchedulePreparer
-from wf_scheduling.store import StaleScheduleRevisionError
+from wf_scheduling.store import ScheduleExistsError, StaleScheduleRevisionError
 
 from .models import (
     JsonProjector,
@@ -207,11 +207,20 @@ class WorkflowScheduleApi:
             }
         )
         self._validate_definition(schedule)
-        stored = store.create_schedule(schedule)
         # Creation never backfills time before the revision: the consumed
-        # watermark starts at creation so catch-up only ever covers
-        # downtime after this revision, never pre-creation instants.
+        # watermark starts at creation so catch-up only ever covers downtime
+        # after this revision, never pre-creation instants. Check the id
+        # before this write so a duplicate create cannot overwrite the
+        # existing schedule's watermark; publishing the schedule last keeps
+        # a failed watermark write invisible to the poller.
+        try:
+            store.get_schedule(schedule_id)
+        except KeyError:
+            pass
+        else:
+            raise ScheduleExistsError(f"schedule id already exists: {schedule_id!r}")
         store.save_consumed(schedule_id, now)
+        stored = store.create_schedule(schedule)
         return _PROJECT_SCHEDULE(stored.model_dump(mode="json"))
 
     async def get_schedule(self, *, schedule_id: str) -> ScheduleResult: