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/db/migrations/main/001_schema.sql

@@ -46,7 +46,7 @@ CREATE TABLE webhooks (
     user_id uuid NOT NULL,
     name text NOT NULL,
     description text,
-    retention_days integer DEFAULT 30,
+    retention_days integer,
     PRIMARY KEY (id),
     CONSTRAINT fk_users_webhooks FOREIGN KEY (user_id) REFERENCES users (id)
 );