Old formula: COP = 5.0 - (T1-T4)*0.05  (rough estimate, not a measurement)
New formula: COP = (flow_lh × 1.163 × (TW_out - TW_in)) / P_el

Where:
- flow_lh = water_flow_register × 22.1 L/h (derived from Sevra 8kW nominal 1370 L/h)
- TW_in/TW_out from RS485 registers
- P_el = outdoor_unit_current × outdoor_unit_voltage

Also added sensor.moc_cieplna_pompy [W] for thermal power output.

Note: SCOP from power_output_lo register is NOT thermal COP - it measures
compressor shaft power ratio, not heat output. Left as-is with comment.

Current result: COP=5.42 at T4=6°C, TW_in=27, TW_out=31, freq=57Hz (plausible)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>