fix(celery): write display_power value and TTL atomically

- a soft-limit signal between SET and EXPIRE could leave display_power
  without a TTL (stale value that never expires)
- use a single SET with ex= so the write and TTL are atomic

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/anthias_server/celery_tasks.py

@@ -318,8 +318,11 @@ def get_display_power() -> None:
     # — and the on/off state now actually populates instead of only
     # the error cases ever landing.
     try:
-        r.set('display_power', str(diagnostics.get_display_power()))
-        r.expire('display_power', 3600)
+        # Single SET with ex= so the value and its TTL are written
+        # atomically — a soft-limit signal landing between a separate
+        # SET and EXPIRE would otherwise leave the key without a TTL
+        # (a stale display_power that never expires).
+        r.set('display_power', str(diagnostics.get_display_power()), ex=3600)
     except SoftTimeLimitExceeded:
         # The CEC query is meant to be bounded by its own
         # subprocess timeout, but a child wedged in libcec can keep

tests/test_celery_tasks.py

@@ -194,11 +194,13 @@ def test_get_display_power_writes_redis_as_str(
     ):
         get_display_power.apply()
 
-    fake_redis.set.assert_called_once_with('display_power', expected)
+    # Value and TTL are written atomically in a single SET (ex=) so a
+    # soft-limit signal can't leave the key without a TTL.
+    fake_redis.set.assert_called_once_with('display_power', expected, ex=3600)
+    fake_redis.expire.assert_not_called()
     # No bool ever reaches redis — guards against the DataError.
     written = fake_redis.set.call_args.args[1]
     assert isinstance(written, str)
-    fake_redis.expire.assert_called_once_with('display_power', 3600)
 
 
 def test_send_telemetry_task_dispatches() -> None: