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/config_validation.py

@@ -0,0 +1,22 @@
+from datetime import datetime as datetime_sys
+
+import voluptuous
+
+
+def string(value):
+    if value is None:
+        raise voluptuous.Invalid("string value is None")
+    return str(value)
+
+
+def datetime(value):
+    if isinstance(value, datetime_sys):
+        return value
+    try:
+        return datetime_sys.fromisoformat(value)
+    except (TypeError, ValueError) as err:
+        raise voluptuous.Invalid("invalid datetime") from err
+
+
+def config_entry_only_config_schema(domain):
+    return lambda config: config