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/const.py

@@ -0,0 +1,29 @@
+from enum import Enum
+
+CONF_EMAIL = "email"
+CONF_PASSWORD = "password"
+
+
+class Platform(str, Enum):
+    CALENDAR = "calendar"
+    SENSOR = "sensor"
+
+
+class UnitOfLength(str, Enum):
+    CENTIMETERS = "cm"
+
+
+class UnitOfMass(str, Enum):
+    KILOGRAMS = "kg"
+
+
+class UnitOfTemperature(str, Enum):
+    CELSIUS = "°C"
+
+
+class UnitOfTime(str, Enum):
+    MINUTES = "min"
+
+
+class UnitOfVolume(str, Enum):
+    MILLILITERS = "mL"