fix: use real elapsed time for energy calculation instead of fixed in…

[derolli1976]

…terval

In WebSocket mode, push updates arrive more frequently than the configured polling interval (e.g. every 15s vs 60s). The energy calculation was using the fixed configured interval for every update, causing energy to be overcounted by a factor of (interval / real_elapsed).

Fix: Track the actual timestamp of each update and use the real elapsed time between consecutive updates for the Riemann sum integration. The configured interval is only used as fallback for the very first update after start/restore.

Fixes overcounted Energy produced today (DC) sensor values in WebSocket mode.

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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)

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• 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.