Read Teslemetry backup reserve from a local Powerwall

[Bre77]

Proposed change

Third rung of the local Powerwall data-reading stack for Teslemetry, on top of PW-2.

Routes the backup reserve number's read-back to the local Powerwall on a paired site, via a small local config.json coordinator. The raw site_info.backup_reserve_percent is scaled to the user-facing percentage with aiopowerwall's raw_to_scaled_reserve(). The existing local-first write path (with cloud fallback) is unchanged.

Off-grid vehicle charging reserve stays cloud-only, as aiopowerwall has no local equivalent for it. No entities are added.

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• Dependency upgrade
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