Session: add reference price and CO2

[naltatis]

addresses #11160

Snapshot average grid price and CO2 intensity (next 24h) when a charging session is created.
Adds referencePricePerKWh and referenceCo2PerKWh to the session DB schema and JSON/CSV response. This provides stable reference values for future savings calculations instead of relying on volatile forecast data.

Existing sessions get NULL via GORM auto-migration.

Next steps: (separate PRs)

• Add a UI option to (bulk-)backfill reference data for old charging sessions.
• Replace the static savings calculation (hard coded region values) and use the new data fields.

[sourcery-ai]

Hey - I've found 1 issue, and left some high level feedback:

• The AverageRate helper currently computes a simple arithmetic mean of all overlapping rates, which biases the result if the underlying rate periods have different durations; consider making this a duration-weighted average over the [now, now+d) interval instead.
• AverageRate hardcodes time.Now() inside the function, which makes behavior time-dependent and harder to test; passing the start time as a parameter (or injecting a clock) would make the function more predictable and reusable.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The `AverageRate` helper currently computes a simple arithmetic mean of all overlapping rates, which biases the result if the underlying rate periods have different durations; consider making this a duration-weighted average over the [now, now+d) interval instead.
- `AverageRate` hardcodes `time.Now()` inside the function, which makes behavior time-dependent and harder to test; passing the start time as a parameter (or injecting a clock) would make the function more predictable and reusable.

## Individual Comments

### Comment 1
<location path="tariff/tariffs.go" line_range="47-56" />
<code_context>
 	return rr
 }

+// AverageRate returns the arithmetic mean of rates in [now, now+d), or nil if unavailable.
+func AverageRate(t api.Tariff, d time.Duration) *float64 {
+	rr := Rates(t)
+	if rr == nil {
+		return nil
+	}
+
+	now := time.Now()
+	end := now.Add(d)
+
+	var sum float64
+	var count int
+	for _, r := range rr {
+		if r.Start.Before(end) && r.End.After(now) {
+			sum += r.Value
+			count++
+		}
+	}
+
+	if count == 0 {
+		return nil
+	}
+
+	avg := sum / float64(count)
+	return &avg
+}
</code_context>
<issue_to_address>
**suggestion:** Consider weighting the average by time instead of counting overlapping rate entries equally.

Right now each overlapping rate contributes equally to the mean, regardless of how long it applies within `[now, now+d)`. If rate periods can vary a lot in length, this can significantly skew the result (e.g. brief spikes vs long stable periods). You may want to compute a time‑weighted average instead: for each overlapping rate, accumulate `r.Value * overlapDuration` and then divide the total by the sum of all overlap durations.
</issue_to_address>

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