v0.5.0: peak and off-peak prices, payback date

[PeterSlijkhuis]

Before: Battery ROI required a dynamic price sensor, so people on a fixed or peak/off-peak contract couldn't use it. When the price sensor was unavailable, no profit was booked. Payback was shown only as a number of years.

After:

• The price sensor stays first and primary. It is now optional when a Peak price is entered (all-in, per kWh). On a single-rate contract, the peak price is the only one to fill in.
• Peak/off-peak contracts can add an Off-peak price with its hours (default 23:00 to 07:00). An Off-peak all weekend switch, off by default, covers contracts where Saturday and Sunday are fully off-peak.
• With both a sensor and a peak price, the sensor wins and the typed-in prices fill in while the sensor is unavailable.
• Setup refuses to save without either a price sensor or a peak price.
• The payback sensor gets a paid_back_on attribute (the expected date). The card shows "Paid back by Mar 2031 (4.5 y)".

How: _build falls back to _fixed_price(local time) when the sensor gives no value. Surcharge and VAT still apply only to the sensor price, because typed-in prices are all-in. The same path is used when replaying history. Version 0.5.0.

Tests: 26 pass, including peak/off-peak and weekend switching, the no-price error, the sensor-unavailable fallback and the payback date.

[PeterSlijkhuis]

The new hacs check fails on 2 of 9 validations: the repository has no description and no topics. Both are repository settings (About section on the repo page), not code, so they can't be fixed from this PR. Everything else passes, including brands, the manifest and hacs.json. The check turns green on the next run once the settings are filled in.