Add audit log for moderation and admin actions

New audit_log table records actor, action, target and a JSON snapshot
of the details, so entries stay readable after the target is deleted;
the actor is set to NULL when the user is removed. logAudit() is called
from upload delete (single and bulk), comment delete, tag removal, role
create/delete, role permission changes, role assign/revoke and MOTD
updates. Logging failures only warn and never fail the action itself.

The admin page gets an audit card behind the new audit.view permission
with action filter, click-to-filter by user, readable summaries per
action and server-side pagination. Action catalog lives in
shared/utils/audit.ts, labels under admin.auditActions (de/en).

Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

app/pages/admin.vue

@@ -331,6 +331,15 @@ async function saveMotd() {
         </form>
       </CardContent>
     </Card>
+    <Card v-if="can('audit.view')" class="lg:col-span-2">
+      <CardHeader>
+        <CardTitle>{{ $t('admin.audit.title') }}</CardTitle>
+        <CardDescription>{{ $t('admin.audit.desc') }}</CardDescription>
+      </CardHeader>
+      <CardContent>
+        <AuditLog />
+      </CardContent>
+    </Card>
     <p v-if="errorMessage" class="text-sm text-destructive lg:col-span-2">
       {{ errorMessage }}
     </p>