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>

custom_components/babytracker/entity.py

@@ -0,0 +1,48 @@
+"""Base entity for a baby."""
+
+from __future__ import annotations
+
+from homeassistant.core import callback
+from homeassistant.helpers.device_registry import DeviceEntryType, DeviceInfo
+from homeassistant.helpers.update_coordinator import CoordinatorEntity
+
+from .babytracker_api import Baby
+from .const import DOMAIN
+from .coordinator import BabyTrackerCoordinator
+
+
+class BabyEntity(CoordinatorEntity[BabyTrackerCoordinator]):
+    """An entity on a baby's device."""
+
+    _attr_has_entity_name = True
+
+    def __init__(self, coordinator: BabyTrackerCoordinator, baby: Baby, key: str) -> None:
+        super().__init__(coordinator)
+        self.baby_id = baby.id
+        self._attr_unique_id = f"{baby.id}_{key}"
+        self._attr_translation_key = key
+        self._attr_device_info = DeviceInfo(
+            identifiers={(DOMAIN, baby.id)},
+            name=baby.name,
+            manufacturer="Baby Tracker",
+            entry_type=DeviceEntryType.SERVICE,
+        )
+
+    @property
+    def available(self) -> bool:
+        return super().available and self.baby_id in self.coordinator.data.babies
+
+
+def async_add_baby_entities(entry, coordinator: BabyTrackerCoordinator, async_add_entities, create) -> None:
+    """Add entities for each baby now, and for babies that appear later."""
+    known: set[str] = set()
+
+    @callback
+    def add_new() -> None:
+        new = [baby for baby_id, baby in coordinator.data.babies.items() if baby_id not in known]
+        if new:
+            known.update(baby.id for baby in new)
+            async_add_entities([entity for baby in new for entity in create(baby)])
+
+    add_new()
+    entry.async_on_unload(coordinator.async_add_listener(add_new))