Offer archive expiry choices on the target forms, show plain units (closes #396)

Adding or editing a database target now offers the archive expiry
choices of the new webhook page (never, 1h, 12h, 24h, 30d, 90d, 365d)
in place of a text field. The list is defined once, in
internal/handlers/archive_expiry.go, and all three forms render it. The
edit form starts on the stored expiry; one that is not among the choices
is listed first as its own entry, so saving unchanged keeps it. The
target list shows the expiry in plain units, such as "30 days" or
"12 hours", or "never".

Model: opus-5-5

templates/target_edit.html

@@ -60,8 +60,12 @@
             {{if eq .Target.Type "database"}}
             <div class="form-group">
                 <label for="expiry" class="label">Archive Expiry</label>
-                <input type="text" id="expiry" name="expiry" value="{{.Target.Config.Expiry}}" placeholder="never" class="input">
-                <p class="text-xs text-gray-500 mt-1">"never" (the default when blank) keeps archived rows forever, or a Go duration like "720h" prunes older rows.</p>
+                <select id="expiry" name="expiry" class="input">
+                    {{range .ArchiveExpiryChoices}}
+                    <option value="{{.Value}}"{{if .Selected}} selected{{end}}>{{.Label}}</option>
+                    {{end}}
+                </select>
+                <p class="text-xs text-gray-500 mt-1">Archived events older than this are deleted from the archive; never keeps them all.</p>
             </div>
             {{end}}