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.

templates/source_detail.html

@@ -181,7 +181,7 @@
 
     <!-- Info -->
     <div class="mt-4 text-sm text-gray-400">
-        <p>Retention: {{.Webhook.RetentionDays}} days &middot; Created: {{.Webhook.CreatedAt.Format "2006-01-02 15:04:05 UTC"}}</p>
+        <p>Retention: {{.Webhook.RetentionLabel}} &middot; Created: {{.Webhook.CreatedAt.Format "2006-01-02 15:04:05 UTC"}}</p>
     </div>
 </div>
 {{end}}

templates/source_edit.html

@@ -28,7 +28,8 @@
 
             <div class="form-group">
                 <label for="retention_days" class="label">Retention (days)</label>
-                <input type="number" id="retention_days" name="retention_days" value="{{.Webhook.RetentionDays}}" min="1" max="365" class="input">
+                <input type="number" id="retention_days" name="retention_days" value="{{.Webhook.RetentionDays}}" min="0" class="input">
+                <p class="text-xs text-gray-500 mt-1">Currently {{.Webhook.RetentionLabel}}. Enter 0 to retain events forever.</p>
             </div>
 
             <div class="flex gap-3">

templates/sources_list.html

@@ -25,7 +25,7 @@
                     <p class="text-sm text-gray-500 mt-1">{{.Description}}</p>
                     {{end}}
                 </div>
-                <span class="badge-info">{{.RetentionDays}}d retention</span>
+                <span class="badge-info">Retention: {{.RetentionLabel}}</span>
             </div>
             <div class="flex gap-6 mt-4 text-sm text-gray-500">
                 <span>{{.EntrypointCount}} entrypoint{{if ne .EntrypointCount 1}}s{{end}}</span>

templates/sources_new.html

@@ -28,8 +28,8 @@
 
             <div class="form-group">
                 <label for="retention_days" class="label">Retention (days)</label>
-                <input type="number" id="retention_days" name="retention_days" value="30" min="1" max="365" class="input">
-                <p class="text-xs text-gray-500 mt-1">How long to keep event data.</p>
+                <input type="number" id="retention_days" name="retention_days" value="{{.DefaultRetentionDays}}" min="0" class="input">
+                <p class="text-xs text-gray-500 mt-1">How long to keep event data. Enter 0 to retain events forever.</p>
             </div>
 
             <div class="flex gap-3">