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 expiry it shows: the stored one, or the
submitted one after a refused save. 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/source_detail.html

@@ -203,8 +203,15 @@
                     <template x-if="isDatabase">
                         <div>
                             <input type="hidden" name="type" value="database">
-                            <input type="text" name="expiry" :value="expiry" placeholder="never" class="input text-sm">
-                            <p class="text-xs text-gray-500 mt-1">Archive expiry: "never" (default) keeps rows forever, or a duration like "720h" prunes older rows.</p>
+                            <div class="flex gap-2 items-center">
+                                <label class="text-sm text-gray-700">Archive expiry:</label>
+                                <select name="expiry" :value="expiry" class="input text-sm w-24">
+                                    {{range .ArchiveExpiryChoices}}
+                                    <option value="{{.Value}}">{{.Label}}</option>
+                                    {{end}}
+                                </select>
+                            </div>
+                            <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>
                     </template>
                     <template x-if="isLog">