Meter (Home Assistant): support return energy entity

[github-actions]

fixes #31856

The homeassistant meter template only exposed a single energy entity, even though its own help text promised it covers both "consumed/produced" energy. There was no way to configure the opposite direction (grid export / battery discharge), because meter/homeassistant.go never decorated the existing api.MeterReturnEnergy capability.

• Add a returnEnergy param to the homeassistant meter template, rendered as returnenergy:
• Decode ReturnEnergy in meter/homeassistant.go and decorate implement.MeterReturnEnergy when set, mirroring the existing energy/MeterEnergy handling
• Clarify the energy/returnEnergy help text to each describe a single direction instead of "consumed/produced"

🤖 Generated with Claude Code

[naltatis]

LGTM

[sourcery-ai]

Hey - I've left some high level feedback:

• Consider guarding the implement.MeterReturnEnergy decorator with a non-empty cc.ReturnEnergy check (mirroring the energy handling) so that a nil returnEnergyG is never passed in and behavior is explicit rather than relying on implement.May to handle nil.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- Consider guarding the `implement.MeterReturnEnergy` decorator with a non-empty `cc.ReturnEnergy` check (mirroring the `energy` handling) so that a nil `returnEnergyG` is never passed in and behavior is explicit rather than relying on `implement.May` to handle nil.

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[andig]

@naltatis könntest Du die Übersetzungen prüfen? Einspeiseenergieentität ist im Allgemeinen Fall immer falsch.

[naltatis]

Update: we use "Gegenrichtung" / "reverse direction" in ui and docs.