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/database/retention.go

@@ -243,7 +243,8 @@ func (r *RetentionReaper) reapWebhook(
 
 // retentionCutoff returns the timestamp before which a webhook's
 // events have expired, and whether any cutoff applies at all. It
-// reports false for a retain-forever policy, so no DELETE is issued.
+// reports false for a retention of 0, which means retain forever, so
+// no DELETE is issued.
 //
 // The day count is clamped to MaxFiniteRetentionDays first. This is
 // defense in depth rather than decoration: a time.Duration is an int64