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.

TODO.md

@@ -28,6 +28,12 @@ databases currently grow without bound.
 
 # Completed Steps
 
+- 2026-08-09 Make retain-forever reachable from the normal create and
+  edit flows (#79): a `RetentionForeverDays = 365 * 1000` sentinel, a
+  `Webhook.BeforeSave` hook rewriting any non-positive `retention_days`
+  to it ahead of GORM's own column defaulting, a reaper that skips such
+  webhooks outright, form validation that honours `0` and rejects
+  garbage with a 400, and a retention UI that says "forever"
 - 2026-08-07 Update golangci-lint to v2.12.2 (Docker image digest in
   `Dockerfile`, release-archive sha256 pins in `script/bootstrap`),
   adopt the canonical `.golangci.yml` (v2 `linters.settings` layout so