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

RetentionDays carried gorm:"default:30", so GORM substituted 30 for a
zero value while building the insert. A webhook could therefore never
be configured to keep its events indefinitely: the reaper's
retain-forever branch existed but was unreachable from the normal
create and edit flows.

Introduce database.RetentionForeverDays = 365 * 1000 as the sentinel
for "retain forever" and a Webhook.BeforeSave hook that rewrites any
non-positive RetentionDays to it. The rewrite has to live in the hook
rather than at the call sites: GORM applies the column default while
converting the model to insert values, which happens after BeforeSave,
so anything later loses that race. Putting it on the model also means
a future call site, such as the planned REST API, cannot bypass it.

The reaper now skips a webhook when Webhook.RetainsForever reports
true, which recognises the sentinel and keeps honouring the old <= 0
values for rows written before it existed. Without this the sentinel,
being positive, would have produced a cutoff a thousand years in the
past and a DELETE matching nothing on every sweep.

Form handling is shared by create and edit through parseRetentionDays
so the two cannot drift: an empty field keeps the previous behaviour
(default on create, unchanged on edit), 0 is honoured, and an
unparseable or negative value is a 400 that re-renders the form rather
than a silently substituted default.

The retention inputs drop max="365". That cap was not cosmetic: the
edit form pre-fills the stored value, so a retain-forever webhook
rendered 365000 into an input capped at 365 and browser validation
would have blocked saving any edit to it. min becomes 0 with a hint
explaining what 0 does, and the list and detail views render a
RetentionLabel of "forever" instead of a raw day count.

The 30-day default is consolidated into database.DefaultRetentionDays,
referenced from the handler and from the create form's pre-filled
value, with a test asserting it agrees with the struct tag that cannot
reference it.

internal/database/retention.go

@@ -114,7 +114,8 @@ func (r *RetentionReaper) run(ctx context.Context) {
 }
 
 // sweep lists every webhook from the main database and reaps expired
-// rows from each per-webhook database whose RetentionDays is positive.
+// rows from each per-webhook database that has a finite retention
+// policy. Webhooks set to retain forever are skipped entirely.
 func (r *RetentionReaper) sweep(ctx context.Context) {
 	var webhooks []Webhook
 
@@ -139,8 +140,13 @@ func (r *RetentionReaper) sweep(ctx context.Context) {
 
 		wh := webhooks[i]
 
-		// RetentionDays of zero or less means retain forever.
-		if wh.RetentionDays <= 0 {
+		// Skip retain-forever webhooks before building any query.
+		// RetainsForever covers both the RetentionForeverDays
+		// sentinel and the non-positive values that predate it: the
+		// sentinel is a positive number, so without this the reaper
+		// would compute a cutoff a thousand years in the past and
+		// issue a DELETE matching nothing on every single sweep.
+		if wh.RetainsForever() {
 			continue
 		}