fix(entity): replace deprecated via_device with via_device_id

[C3H3-AI]

Problem

Home Assistant logs a deprecation warning on every setup:

WARNING (MainThread) [homeassistant.helpers.frame] Detected that custom
integration 'hotata' calls `device_registry.async_get_or_create` with a
deprecated `via_device` parameter; use `via_device_id` instead at
custom_components/hotata/entity.py, line 74
This will stop working in Home Assistant 2027.8.0

DeviceInfo.via_device takes an identifier tuple, which the device registry
no longer accepts. The parameter is removed in HA 2027.8, so on that release
the gateway relationship silently disappears.

Why this is not a one-line swap

via_device_id needs the device registry id of the gateway, and that id
does not exist when __init__ builds DeviceInfo: entities are constructed
before the registry has finished processing, and the gateway device may not be
registered yet. Passing an id that early would leave the relationship
permanently unset.

Fix

Move the link into async_added_to_hass, which runs after both the entity's
own device and its gateway are registered:

async def async_added_to_hass(self) -> None:
    await super().async_added_to_hass()

    parent_iot_id = self.device.parent_iot_id
    if not parent_iot_id:
        return

    registry = dr.async_get(self.hass)

    parent = registry.async_get_device(identifiers={(DOMAIN, parent_iot_id)})
    if parent is None:
        return   # gateway not registered: stay top-level, never dangle

    own = registry.async_get_device(identifiers={(DOMAIN, self.device.iot_id)})
    if own is None or own.via_device_id == parent.id:
        return   # already linked

    registry.async_update_device(own.id, via_device_id=parent.id)

Three guards keep it safe:

Condition	Behaviour
No parent_iot_id	Return immediately — standalone devices never touch the registry
Gateway not registered	Leave via_device_id unset: a parentless device renders at the top level, whereas a dangling id is dropped silently
Already linked	No-op, so repeated setups do not churn the registry

Behaviour change

None for standalone devices. They report an empty parent_iot_id, so the
hook returns before doing any registry work — the branch never executes.

For gated devices the outcome is the same as before (child under gateway), just
through the supported API.

Testing

• Every module under custom_components/hotata/ parses cleanly
• via_device= no longer appears anywhere in the tree
• Deployed against a live HA 2026.9.1 instance: the warning is gone and the
  integration sets up with zero errors