Store a webhook's retention as a plain day count, 0 meaning forever (closes #539)

retention_days loses its column default of 30, and Webhook.RetentionDays
its default tag, so a 0 is stored as 0. The 365000-day stand-in
RetentionForeverDays and the BeforeSave hook that wrote it are gone. The
reaper and the "forever" label read 0 as forever, and the forms accept 0
or 1 to MaxFiniteRetentionDays; a blank field still gives 30 on the new
webhook form and keeps the stored value on the edit form. A negative
value, which the forms refuse, is still read as forever so it cannot put
the reaper's cutoff in the future.

Model: opus-5-5

internal/handlers/webhook_stats_test.go

@@ -500,7 +500,7 @@ func TestWebhookStats_PaneShowsRetentionPeriod(t *testing.T) {
 		want          string
 	}{
 		{30, "30 days"},
-		{database.RetentionForeverDays, "forever"},
+		{0, "forever"},
 	}
 
 	for _, tt := range tests {