fix(crd): persist status.emptySince — the field was pruned away by the schema

The operator stamps EmptySince to time the idle auto-stop window, but the CRD's
status schema never declared it. Kubernetes pruned the field on every write
(apiserver warning: unknown field "status.emptySince"), so the timer reset to
nil on every read and idle auto-stop could never fire — a defect invisible to
the fake-client unit tests, which do not enforce the CRD schema. Found live:
the stamp was silently dropped the moment it was set.

Schema now declares emptySince (date-time) like its sibling timestamps.

deploy/crd/felis.lolicon.best_minecraftservers.yaml

@@ -347,6 +347,14 @@ spec:
                   - type
                   type: object
                 type: array
+              emptySince:
+                description: |-
+                  EmptySince is when the operator first observed 0 online players during
+                  a Running phase (spec §8 idle auto-stop). It is reset when a player joins
+                  or the server stops, so the empty-duration counter starts fresh each time
+                  the server becomes unoccupied.
+                format: date-time
+                type: string
               endpoint:
                 description: Endpoint is where the proxy should route traffic.
                 properties: