Add state_class: measurement for battery level sensors (+ per-entity overrides) to enable statistics

[derolli1976]

Description

This PR classifies the battery level entities as measurement so they are recorded by Home Assistant’s Recorder and become available in Long-Term Statistics and Statistics cards.

• Adds a small STATE_CLASS_OVERRIDES map to target specific entities (e.g., sensor.energy_battery_charge_level and unit-specific variants).
• Applies overrides in the entity factory so SoC (%) is exposed with state_class=measurement.
• No entity IDs or unique_ids are changed. No configuration changes required.

Motivation and Context

Users want to track daily min/max/avg battery state of charge to assess charge/discharge windows and decide on potential capacity upgrades. Without a state_class, these sensors aren’t included in Recorder and can’t power Statistics cards.

Closes #54

How Has This Been Tested?

Local test in Home Assistant:

1. Update to this version, reload the integration (or restart HA).
2. Verify in Developer Tools → States that the battery SoC entity now shows state_class: measurement and the correct device_class/unit.
3. Add a Statistics card for the SoC sensor; confirm min/max/mean values appear after new samples are collected.
4. Confirm no unit changes occur over time and values remain numeric.

Verified no migration or unique_id changes; history builds from installation time onward.
Confirmed behavior when the entity is excluded from Recorder (expected: no stats until exclusion removed).

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.