Forecast: make solar adjustment a server-side setting

[andig]

fixes #31832

Promotes the solar forecast adjustment from a browser-local UI preference to a server-side setting, per @naltatis' suggestion in the issue.

The "adjust to real production data" switch was stored only in localStorage (settings_solar_adjusted), so it silently reset whenever a browser dropped site storage, and it was per-client rather than per-installation. Meanwhile the optimizer applied the scale factor unconditionally, so the switch never matched what the optimizer actually did.

• new solarAdjusted site setting, persisted in the settings table and published to the UI
• POST /api/solaradjusted/{enable} plus a solarAdjusted MQTT setter
• optimizer applies the scale factor only when the setting is enabled (effectiveSolarScale)
• frontend reads the setting from the store instead of localStorage

forecast.solar.scale is still published unconditionally and is now always present, carrying 1 when unscaled. This keeps the percentage label on the toggle meaningful while the setting is off.

Default is off, matching the previous UI default. Note this changes optimizer behaviour: it no longer scales the solar forecast unless the setting is enabled.

Per-forecast granularity (as raised in the issue) is not included, since the scale value is calculated on totals.

🤖 Generated with Claude Code

[sourcery-ai]

Hey - I've left some high level feedback:

• The changeAdjusted handler now only posts to the API and logs errors, but does not revert the checkbox state on failure, which can leave the UI out of sync with the actual server-side setting; consider either optimistic updating with rollback on error or re-reading solarAdjusted from the store after the request.
• The new solarAdjusted flag is used in Energyflow.vue via a prop, but it’s only added as a prop to Site.vue here; double-check that all parent components wiring Energyflow pass the correct solarAdjusted value from the store so the forecast scaling behavior stays consistent across views.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The `changeAdjusted` handler now only posts to the API and logs errors, but does not revert the checkbox state on failure, which can leave the UI out of sync with the actual server-side setting; consider either optimistic updating with rollback on error or re-reading `solarAdjusted` from the store after the request.
- The new `solarAdjusted` flag is used in `Energyflow.vue` via a prop, but it’s only added as a prop to `Site.vue` here; double-check that all parent components wiring `Energyflow` pass the correct `solarAdjusted` value from the store so the forecast scaling behavior stays consistent across views.

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[naltatis]

😆 I have a mostly identical unpublished PR locally.