perf(ownership): Fetch only the latest assignment activity (#128144)

Autoassignment checked `if activity:` on the assignment history
queryset, which loaded every ASSIGNED activity for the group just to
read the newest one. Use `.first()` so it only fetches one row.

Same query count, but it was returning about 78M rows/day in US for 38M
executions, so roughly half of those rows go away, pretty small thing.
Also adds `-id` as a tiebreaker, matching the smart assignment scoring
query.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: scttcper

src/sentry/models/projectownership.py

@@ -342,11 +342,13 @@ def handle_auto_assignment(
 
             logging_extra = {**logging_extra, **activity_details}
 
-            activity = Activity.objects.filter(
-                group=group, type=ActivityType.ASSIGNED.value
-            ).order_by("-datetime")
-            if activity:
-                auto_assigned = activity[0].data.get("integration")
+            latest_assignment = (
+                Activity.objects.filter(group=group, type=ActivityType.ASSIGNED.value)
+                .order_by("-datetime", "-id")
+                .first()
+            )
+            if latest_assignment:
+                auto_assigned = latest_assignment.data.get("integration")
                 if not auto_assigned and not force_autoassign:
                     return