Make daily traffic scale to dozens of screens: incremental sync, ping keepalive, one action per message (0.2.39)

App 0.2.39 / firmware 0.2.33, items 1, 2, 4, 5 and 6 of the 30-screen scaling check.

- Only the tiles (and top bar) of changed entities are rebuilt; the screen list is
  cached until the registry or a screen diagnostic changes; the SSE stream follows.
- Sensor history comes from one recorder/statistics_during_period call per window in
  a background task, with the REST history as fallback per sensor without statistics.
- Firmware 0.2.33 answers a keepalive ping with its layout revision; a mismatch or a
  restart asks for the layout again, everything is repeated hourly as a safety net.
- The screen_message API action takes one whole message; the text inbox chunks stay
  as fallback for older firmware. A repeated unchanged layout keeps the inbox state.
- Quiet diagnostics: Opgestart (uptime as timestamp) replaces Uptime, heap every
  5 min, setting numbers publish only on change.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

tests/test_updates.py

@@ -137,7 +137,7 @@ class UpdaterTests(unittest.IsolatedAsyncioTestCase):
                       'text.fw1': {'state': '0.2.16'}, 'text.screen2': {'state': 'Ready'}, 'text.fw2': {'state': '0.2.16'}}
             changed = asyncio.Event()
             def __init__(self): self.messages, self.calls = [], []
-            async def send(self, inbox, message): self.messages.append((inbox, message))
+            async def send(self, inbox, message, action=None): self.messages.append((inbox, message))
             async def request(self, kind, **data): self.calls.append((kind, data))
         manager = Manager(HA(), Path(path) / 'screens.json')
         manager.firmware = FakeFirmware(manager.ha, outcome)