Zero stale optimizer readings when panels stop reporting at night

[Copilot]

When optimizers sleep at sunset they stop emitting power reports, so the coordinator's last PowerReportEvent stays in data["nodes"][barcode] and sensors show frozen daytime power/voltage/current all night.

Changes

• Stale-data timer (coordinator.py) — added _schedule_stale_check/_perform_stale_check, scheduled and cancelled alongside the existing midnight-reset timer. Every STALE_CHECK_INTERVAL_SECONDS (30s) it inspects each node's last_update; nodes silent longer than STALE_DATA_TIMEOUT_SECONDS (120s, aligned with ENERGY_GAP_THRESHOLD_SECONDS) get their instantaneous values zeroed and a coordinator update pushed.
• Zeroed fields (const.py STALE_ZERO_KEYS) — power, performance, voltage_in/out, current_in/out, dc_dc_duty_cycle.
• Preserved — cumulative meters (daily_energy_wh, total_energy_wh, readings_today) and diagnostics (temperature, rssi); zeroing 0 °C / 0 dBm would be misleading rather than helpful.
• Tests — 6 cases covering zeroing, fresh-node no-op, already-zero no-op, missing/None last_update skip, reschedule, and stop-cancel.
• Docs — README feature bullet + CHANGELOG entry.

Updates are idempotent: nodes already at zero, recently reporting, or lacking a last_update produce no coordinator update.

Notes for review

• 120s is a deliberate balance — long enough to ignore transient daytime gaps, short enough to reflect sleep promptly. Could be made configurable via the options flow later (see planning/future_considerations.md item 8).
• Aggregate string/installation sensors recompute automatically from the zeroed node values via the pushed update.