Tariffs: update Pstryk API integration for pricing metrics

[redzioch]

Fix #28982

Pstryk changed their API since 01.04.2026.

Endpoints used in evcc no longer exist. Updated tariff template to match updated API.

Tested with newest stable binary (https://github.com/evcc-io/evcc/blob/411f194645b522e6221fd7d588e8b57f8c59e628/CONTRIBUTING.md#device-templates) - works.

[sourcery-ai]

Hey - I've left some high level feedback:

• The window_start and window_end query parameters build slightly different timestamp formats (2006-01-02T15 vs 2006-01-02), which may lead to inconsistent or invalid ISO8601 values; consider aligning both to the same YYYY-MM-DDTHH:MM:SSZ pattern.
• The jq filter uses select(.metrics.pricing != null) which will error if metrics is missing; consider a safer pattern like select((.metrics // {}) | .pricing != null) to make the parsing more robust to partial responses.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The `window_start` and `window_end` query parameters build slightly different timestamp formats (`2006-01-02T15` vs `2006-01-02`), which may lead to inconsistent or invalid ISO8601 values; consider aligning both to the same `YYYY-MM-DDTHH:MM:SSZ` pattern.
- The `jq` filter uses `select(.metrics.pricing != null)` which will error if `metrics` is missing; consider a safer pattern like `select((.metrics // {}) | .pricing != null)` to make the parsing more robust to partial responses.

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[andig]

Solves #28982

@redzioch GH doesn't recognize this. Working must be "Fix(es)" for linking to work ;)