feat(filters): date-range filtering on PlayEventFilter.started/ended (#67)

Change PlayEventFilter.started/ended from StringCriterion to DateCriterion
so they support GREATER_THAN / LESS_THAN / BETWEEN, enabling
"finished in year Y" to be expressed through the filter system.

PlayEvent.started/ended are DateField columns, so the criteria apply with
bare field names (no __date lookup, unlike SessionFilter.timestamp_start
which is a datetime). This mirrors the existing PurchaseFilter DateField
precedent. Deserialization auto-switches via the field annotation and the
serialized JSON shape is unchanged, so the type change is backward-compatible.

Prerequisite for #65 Tier-2 stats-page filtered links. Part of #61.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

games/filters.py

@@ -899,8 +899,8 @@ class PlayEventFilter(OperatorFilter):
     NOT: PlayEventFilter | None = None
 
     game: MultiCriterion | None = None  # filters on game_id
-    started: StringCriterion | None = None  # date string
-    ended: StringCriterion | None = None  # date string
+    started: DateCriterion | None = None  # DateField, bare lookup
+    ended: DateCriterion | None = None  # DateField, bare lookup
     days_to_finish: IntCriterion | None = None
     note: StringCriterion | None = None
     created_at: StringCriterion | None = None