fix frozen time bounds on the recordings API (#24121)

`after` and `before` defaulted to `datetime.now()` in the function signature, so they were evaluated once at import. Requests that omitted them got a window ending at process start. They now resolve in the handler.

docs/static/frigate-api.yaml

@@ -7197,13 +7197,17 @@ paths:
           in: query
           required: false
           schema:
-            type: number
+            anyOf:
+              - type: number
+              - type: 'null'
             title: After
         - name: before
           in: query
           required: false
           schema:
-            type: number
+            anyOf:
+              - type: number
+              - type: 'null'
             title: Before
       responses:
         '200':