Add Baby Tracker client and Home Assistant integration

Python client for the Baby Tracker sync server, reverse engineered from
the Android app (4.53), and a HACS-installable Home Assistant integration
built on it. Each baby gets a log calendar and summary sensors, and actions
add entries that sync back to the group.

Reading has been run against the real server. Uploading and running inside
real Home Assistant have not.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/ha_stubs/homeassistant/helpers/device_registry.py

@@ -0,0 +1,30 @@
+from dataclasses import dataclass
+from enum import Enum
+
+
+class DeviceEntryType(Enum):
+    SERVICE = "service"
+
+
+def DeviceInfo(**kwargs):
+    return kwargs
+
+
+@dataclass
+class DeviceEntry:
+    identifiers: set
+
+
+class DeviceRegistry:
+    def __init__(self):
+        self.devices = {}
+
+    def async_get(self, device_id):
+        return self.devices.get(device_id)
+
+
+REGISTRY = DeviceRegistry()
+
+
+def async_get(hass):
+    return REGISTRY