Reject a retention duration longer than about 292 years (closes #289)

parseDuration multiplied each number by the length of its unit with no
limit, so from 293y (or 15251w, 3559mo, 106752d, or a sum such as
200y200y) the result wrapped to a negative duration. The purge cutoff
then lay in the future and every snapshot of the name was selected, for
--older-than and --keep-newer-than alike. Each part is now checked
before it is added, and a total past the largest time.Duration is
rejected as an invalid duration, so nothing is deleted. Go units were
already safe: time.ParseDuration rejects an overflow.

With --prune the backup still runs before the duration is read; only
the prune step fails.

Model: opus-5-5

internal/vaultik/helpers_test.go

@@ -111,6 +111,17 @@ func TestParseDuration(t *testing.T) {
 		{"30 days", 0, true},
 		{"30 days ago", 0, true},
 		{"x7d", 0, true},
+		// The largest time.Duration is about 292 years. A larger value
+		// must be rejected, not wrapped to a negative one, which would
+		// put the cutoff in the future and select every snapshot.
+		{"292y", 292 * 365 * 24 * time.Hour, false},
+		{"293y", 0, true},
+		{"3559mo", 0, true},
+		{"15251w", 0, true},
+		{"106752d", 0, true},
+		{"2562048h", 0, true},
+		{"200y200y", 0, true},
+		{"292y6mo", 0, true},
 	}
 
 	for _, tt := range tests {