feat: Dashboard-Metriken nach Mailpiler-Vorbild (Uptime, Aktivität, Prognose)

- storage_stats.go (neu): MailActivityStats (60min/24h/7d/30d), StorageEstimateStats
- dashboard_handlers.go: Uptime (/proc/uptime), activity + estimate in System-Stats-Response
- DashboardTab: Uptime in API-Kachel, neue Kacheln "Mail-Eingang" + "Speicherprognose"
- Warnung (Badge "Knapp!") wenn Partition in <90 Tagen voll

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

internal/storage/storage_stats.go

@@ -0,0 +1,88 @@
+package storage
+
+import (
+	"context"
+	"time"
+)
+
+// MailActivity holds mail counts over different time windows.
+type MailActivity struct {
+	Last60Min int64 `json:"last_60_min"`
+	Last24h   int64 `json:"last_24h"`
+	Last7d    int64 `json:"last_7d"`
+	Last30d   int64 `json:"last_30d"`
+}
+
+// StorageEstimate holds a rough prediction of when disk space will run out.
+type StorageEstimate struct {
+	AvgMailsPerDay  float64 `json:"avg_mails_per_day"`
+	AvgMailBytes    int64   `json:"avg_mail_bytes"`
+	DaysUntilFull   int64   `json:"days_until_full"`   // -1 = unknown/infinite
+	ArchiveAgeDays  int64   `json:"archive_age_days"`
+}
+
+// MailActivityStats returns how many mails arrived in the last 60min / 24h / 7d / 30d.
+func (s *Store) MailActivityStats(ctx context.Context) (*MailActivity, error) {
+	if s.db == nil {
+		return &MailActivity{}, nil
+	}
+	now := time.Now()
+	windows := []time.Duration{
+		60 * time.Minute,
+		24 * time.Hour,
+		7 * 24 * time.Hour,
+		30 * 24 * time.Hour,
+	}
+	counts := make([]int64, len(windows))
+	for i, w := range windows {
+		cutoff := now.Add(-w)
+		row := s.db.QueryRow(ctx,
+			`SELECT COUNT(*) FROM emails WHERE received_at >= $1`, cutoff)
+		_ = row.Scan(&counts[i])
+	}
+	return &MailActivity{
+		Last60Min: counts[0],
+		Last24h:   counts[1],
+		Last7d:    counts[2],
+		Last30d:   counts[3],
+	}, nil
+}
+
+// StorageEstimateStats calculates avg mails/day and avg mail size from the DB.
+// freeDiskBytes: free bytes on the archive partition (passed in from caller).
+func (s *Store) StorageEstimateStats(ctx context.Context, freeDiskBytes uint64) (*StorageEstimate, error) {
+	if s.db == nil {
+		return &StorageEstimate{DaysUntilFull: -1}, nil
+	}
+
+	var totalMails, totalBytes int64
+	var firstReceived *time.Time
+	if err := s.db.QueryRow(ctx,
+		`SELECT COUNT(*), COALESCE(SUM(size_bytes), 0), MIN(received_at) FROM emails`,
+	).Scan(&totalMails, &totalBytes, &firstReceived); err != nil || totalMails == 0 {
+		return &StorageEstimate{DaysUntilFull: -1}, nil
+	}
+
+	ageDays := int64(1)
+	if firstReceived != nil {
+		d := time.Since(*firstReceived).Hours() / 24
+		if d > 1 {
+			ageDays = int64(d)
+		}
+	}
+
+	avgMailsPerDay := float64(totalMails) / float64(ageDays)
+	avgMailBytes := totalBytes / totalMails
+
+	daysUntilFull := int64(-1)
+	if avgMailsPerDay > 0 && avgMailBytes > 0 && freeDiskBytes > 0 {
+		daysUntilFull = int64(float64(freeDiskBytes) / (avgMailsPerDay * float64(avgMailBytes)))
+	}
+
+	return &StorageEstimate{
+		AvgMailsPerDay: avgMailsPerDay,
+		AvgMailBytes:   avgMailBytes,
+		DaysUntilFull:  daysUntilFull,
+		ArchiveAgeDays: ageDays,
+	}, nil
+}