Allow retention_days of 0 to mean retain forever (closes #79)

Rewrites retention_days=0 to the RetentionForeverDays sentinel (365 * 1000)
in Webhook.BeforeSave, so the GORM column default cannot win the race. The
reaper skips retain-forever webhooks before building any query.

Also bounds the reaper's cutoff arithmetic: a time.Duration is int64
nanoseconds, so day counts above MaxFiniteRetentionDays (106751) overflowed
and wrapped the cutoff into the future, where created_at < cutoff matched
every row and the sweep deleted everything. parseRetentionDays now rejects
finite values above the ceiling, and retentionCutoff saturates so rows
written by older versions cannot reach it either.

Views render RetentionLabel() rather than the raw sentinel.

internal/database/database_test.go

@@ -18,6 +18,11 @@ const (
 	testVersion = "test"
 	// testContentType is the event content type used in tests.
 	testContentType = "application/json"
+	// testWebhookName is the Webhook.Name used in tests.
+	testWebhookName = "test-webhook"
+	// testForeverLabel is Webhook.RetentionLabel for a retain-forever
+	// webhook.
+	testForeverLabel = "forever"
 )
 
 func setupTestDB(