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/ui_copy_test.go

@@ -184,8 +184,7 @@ func TestEventLogPageIsCalledFullEventLog(t *testing.T) {
 // TestCreateFormRetentionCopyMatchesBehaviour pins the create form's
 // retention copy to what the code does: the reaper permanently deletes
 // events past the cutoff, an empty field falls back to
-// DefaultRetentionDays, and 0 is rewritten to the retain-forever
-// sentinel by Webhook.BeforeSave.
+// DefaultRetentionDays, and 0 means retain forever.
 func TestCreateFormRetentionCopyMatchesBehaviour(t *testing.T) {
 	t.Parallel()
 
@@ -253,10 +252,7 @@ func TestEditFormRetentionCopyMatchesBehaviour(t *testing.T) {
 		"blank means unchanged, not forever",
 	)
 
-	forever := &database.Webhook{
-		Name:          "wh",
-		RetentionDays: database.RetentionForeverDays,
-	}
+	forever := &database.Webhook{Name: "wh", RetentionDays: 0}
 	forever.ID = "wh-2"
 
 	foreverBody := renderPage(