refactor(coordinator): _on_data hook — collapse 4 _async_update_data overrides

[mrebbert]

Summary

Third candidate from the post-0.1.53 refactor audit. Live/Price/Optimization/Notifications each duplicated the same 5-line skeleton:

```python
async def _async_update_data(self) -> T:
data = await super()._async_update_data()
self._side_effect(data)
return data
```

Base coordinator now calls `await self._on_data(data)` after every successful fetch (default is a no-op). Each subclass keeps its existing side-effect method (EMS repair, negative-price bus events, decision events, notification persist + Store) and wires it via a 2-line `_on_data` override.

Net −3 lines but the real win is uniformity: one documented extension point for "post-fetch side effects" instead of four copies of the same wrapper template.

Test plan

• Full suite green (`.venv/bin/pytest -q` → 226 passed).
• mypy + ruff clean.
• Live-deployed on homie; integration loads clean, `sensor.1k5_aktueller_strompreis` refreshes (price coord fires bus events on the new hook), no coordinator errors in the HA error log.

Notes

• No behavioural change: same fetch pipeline, same side effects fire in the same order, same errors surface as `UpdateFailed` via the base try/except.
• No tests reference the removed `_async_update_data` overrides directly — tests exercise the public bus events / repair issues, which still fire.
• Independent of refactor: _safe_fetch helper + price-sensor MRO cleanup #12 (touches coordinator.py only; no overlap).