Add --keep-newer-than flag for rolling retention window

snapshot create --prune now accepts --keep-newer-than <duration> (e.g.
4w, 30d, 6mo) to keep a rolling window of snapshots instead of only
the latest. Supports d/w/mo/y units and combinations (2w3d).

Without --keep-newer-than, --prune still defaults to keep-latest-only.

internal/vaultik/helpers.go

@@ -2,6 +2,7 @@ package vaultik
 
 import (
 	"fmt"
+	"regexp"
 	"strconv"
 	"strings"
 	"time"
@@ -95,18 +96,39 @@ func parseSnapshotName(snapshotID string) string {
 	return strings.Join(parts[1:len(parts)-1], "_")
 }
 
-// parseDuration parses a duration string with support for days
+// parseDuration parses a duration string with support for human-friendly units:
+// d/day/days, w/week/weeks, mo/month/months, y/year/years, plus standard Go
+// duration units (h, m, s).
 func parseDuration(s string) (time.Duration, error) {
-	// Check for days suffix
-	if strings.HasSuffix(s, "d") {
-		daysStr := strings.TrimSuffix(s, "d")
-		days, err := strconv.Atoi(daysStr)
-		if err != nil {
-			return 0, fmt.Errorf("invalid days value: %w", err)
-		}
-		return time.Duration(days) * 24 * time.Hour, nil
+	if d, err := time.ParseDuration(s); err == nil {
+		return d, nil
 	}
 
-	// Otherwise use standard Go duration parsing
-	return time.ParseDuration(s)
+	re := regexp.MustCompile(`(\d+)\s*([a-zA-Z]+)`)
+	matches := re.FindAllStringSubmatch(s, -1)
+	if len(matches) == 0 {
+		return 0, fmt.Errorf("invalid duration: %q", s)
+	}
+
+	var total time.Duration
+	for _, match := range matches {
+		n, err := strconv.Atoi(match[1])
+		if err != nil {
+			return 0, fmt.Errorf("invalid number %q: %w", match[1], err)
+		}
+		unit := strings.ToLower(match[2])
+		switch unit {
+		case "d", "day", "days":
+			total += time.Duration(n) * 24 * time.Hour
+		case "w", "week", "weeks":
+			total += time.Duration(n) * 7 * 24 * time.Hour
+		case "mo", "month", "months":
+			total += time.Duration(n) * 30 * 24 * time.Hour
+		case "y", "year", "years":
+			total += time.Duration(n) * 365 * 24 * time.Hour
+		default:
+			return 0, fmt.Errorf("unknown time unit %q", unit)
+		}
+	}
+	return total, nil
 }