v0.4.0: history from install date, feed-in price, net metering end date

[PeterSlijkhuis]

Before: All totals started at zero on the day Battery ROI was set up. Telling solar charging from grid charging needed a feed-in price sensor, which users on a fixed feed-in tariff don't have. Users whose net metering ends on a known date had to come back and change the settings on that day.

After:

• Battery installed on (under Battery costs) replays Home Assistant's history from that day and fills in Today, This month, Lifetime, energy and payback. A notification says when it's done and from which day data was found.
• Fixed feed-in price, as an alternative to the sensor. Together with grid power, solar that goes into the battery costs the feed-in price that was given up, and grid charging costs the import price.
• Net metering until: an optional date that starts empty and works in any country. Before that day the feed-in price is ignored, so a stored solar kWh is worth the full import price. From that day the feed-in price counts, so the setup only has to be done once.

How: backfill.py loads raw state history (about 10 days by default). For older periods it falls back to hourly long-term statistics means, then replays everything on a one-minute grid through the same Accumulator and Sample logic; the hub's sample building is shared as _build(get, at), so the net metering date also applies to replayed history. Hours without statistics for the battery power sensor are skipped as downtime. Version 0.4.0.

Tests: 23 pass, including replay profit, skipping downtime, solar vs grid charging with a fixed feed-in, and the switch on the net metering end date.