Add energy_price and tax attributes to interval and daily stat sensors:
- Interval sensors (current/next/previous): energy_price and tax from
the specific 15-minute interval
- Daily min/max sensors: energy_price and tax from the extreme interval
- Daily average sensors: energy_price_mean, energy_price_median,
tax_mean, tax_median — matching the existing mean/median pattern
used for the main price attribute
Calculator caches both mean and median for energy/tax using
calculate_median() from utils/average. All new attributes are
excluded from Recorder to prevent database bloat.
Impact: Users can see price composition (spot price vs. taxes) on
all major price sensors. Enables solar feed-in and net metering
automations based on raw energy prices.
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