Refresh forecast up to 3× per day within daylight window

forecast.solar updates estimates as the day progresses, so a single
morning fetch can be stale by afternoon. New behaviour:
- Re-fetch every 4h within 07:00–19:00 window (3 fetches/day)
- Outside the window always serve the cached value
- Free tier limit is 12 req/day — 3 fetches is well within budget
- First startup outside window still fetches if cache is empty

Config: forecast.fetch_interval, fetch_window_start, fetch_window_end

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/config/config.go

@@ -128,12 +128,23 @@ type SeasonConfig struct {
 
 // ForecastConfig holds forecast.solar API parameters.
 type ForecastConfig struct {
-	Enabled     bool    `yaml:"enabled"`
-	Lat         float64 `yaml:"lat"`
-	Lon         float64 `yaml:"lon"`
-	Declination int     `yaml:"declination"` // panel tilt in degrees
-	Azimuth     int     `yaml:"azimuth"`     // degrees from south (south=0, west=90)
-	KWp         float64 `yaml:"kwp"`         // installed peak power
+	Enabled          bool    `yaml:"enabled"`
+	Lat              float64 `yaml:"lat"`
+	Lon              float64 `yaml:"lon"`
+	Declination      int     `yaml:"declination"`   // panel tilt in degrees
+	Azimuth          int     `yaml:"azimuth"`        // degrees from south (south=0, west=90)
+	KWp              float64 `yaml:"kwp"`            // installed peak power
+	FetchInterval    string  `yaml:"fetch_interval"` // how often to re-fetch during the day (e.g. "4h")
+	FetchWindowStart string  `yaml:"fetch_window_start"` // e.g. "07:00" — no fetches before this
+	FetchWindowEnd   string  `yaml:"fetch_window_end"`   // e.g. "19:00" — no fetches after this
+}
+
+func (f *ForecastConfig) FetchIntervalParsed() time.Duration {
+	d, err := time.ParseDuration(f.FetchInterval)
+	if err != nil || d <= 0 {
+		return 24 * time.Hour // safe default: once per day
+	}
+	return d
 }
 
 // EMSConfig holds operational settings for the EMS daemon.