Metrics: track loadpoint enabled state per slot

[andig]

Records how much of each 15min metrics slot a loadpoint was enabled.

A new nullable meters.enabled column stores the time-weighted fraction (0..1) of the slot during which the loadpoint's charging was enabled. It is integrated over time like power (AddEnabled), sampled in the same accumulator interval as the energy update so it shares the slot's timing.

Only loadpoints sample the state — grid/pv/battery/home/forecast entities leave the column null. Recovered downtime-catchup slots also store null, since enabled was not observed during the gap. AutoMigrate adds the column to existing databases; no manual migration.

This only persists the value; wiring it into history/export/CLI is out of scope here.

🤖 Generated with Claude Code

[sourcery-ai]

Hey - I've found 1 issue

Prompt for AI Agents

Please address the comments from this code review:

## Individual Comments

### Comment 1
<location path="core/metrics/accumulator.go" line_range="62-70" />
<code_context>
+
+// EnabledFraction returns the 0..1 fraction of the slot the loadpoint was
+// enabled, or nil when the state was never sampled (non-loadpoint entities).
+func (m *Accumulator) EnabledFraction() *float64 {
+	if m.totalTime <= 0 {
+		return nil
+	}
+	f := m.enabledTime.Seconds() / m.totalTime.Seconds()
+	return &f
+}
</code_context>
<issue_to_address>
**suggestion:** Clamping the enabled fraction into [0,1] would make the metric robust against unexpected time accounting anomalies.

If `enabledTime` ever slightly exceeds `totalTime` (or becomes negative due to subtle timing bugs), this could yield fractions outside [0,1]. Clamping `f` into that range before returning (e.g. `if f < 0 { f = 0 } else if f > 1 { f = 1 }`) would make the result safer for downstream consumers without changing behavior in the normal case.

```suggestion
// EnabledFraction returns the 0..1 fraction of the slot the loadpoint was
// enabled, or nil when the state was never sampled (non-loadpoint entities).
func (m *Accumulator) EnabledFraction() *float64 {
	if m.totalTime <= 0 {
		return nil
	}

	f := m.enabledTime.Seconds() / m.totalTime.Seconds()

	// Clamp into [0,1] to guard against timing anomalies.
	if f < 0 {
		f = 0
	} else if f > 1 {
		f = 1
	}

	return &f
}
```
</issue_to_address>

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[andig]

@naltatis this PR would provide the basis for creating the kind of "enabled over time" chart other HEMSs seem to have.