feat(admin): audit-log viewer

Surface the admin_audit table (written by every mutating admin action but
previously unreadable) as a new /admin/audit tab. New repo::admin_audit::list
(LEFT JOIN users for the actor's username, filter by action/target_kind/actor/
since, at DESC via admin_audit_at_idx) behind GET /v1/admin/audit, mirroring the
crawler history endpoint's paged/clamped idiom.

Frontend: a self-contained AuditTable (target-kind + window + action filters,
expandable JSON payload, AbortController-cancelled fetches, loading/error/empty
states) and shared fmtAgo() in format.ts.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/routes/admin/+layout.svelte

@@ -9,7 +9,8 @@
         { href: '/admin/crawler', label: 'Crawler' },
         { href: '/admin/analysis', label: 'Analysis' },
         { href: '/admin/settings', label: 'Settings' },
-        { href: '/admin/system', label: 'System' }
+        { href: '/admin/system', label: 'System' },
+        { href: '/admin/audit', label: 'Audit' }
     ];
 </script>
 

frontend/src/routes/admin/audit/+page.svelte

@@ -0,0 +1,25 @@
+<script lang="ts">
+    import AuditTable from '$lib/components/admin/AuditTable.svelte';
+</script>
+
+<svelte:head><title>Audit log · Admin</title></svelte:head>
+
+<h1>Audit log</h1>
+<p class="lead">
+    Every admin action — crawler runs, resyncs, settings changes, session
+    updates — is recorded here. Click a row to inspect its payload.
+</p>
+
+<AuditTable />
+
+<style>
+    h1 {
+        margin-bottom: var(--space-2);
+    }
+    .lead {
+        color: var(--text-muted);
+        font-size: var(--font-sm);
+        margin-bottom: var(--space-4);
+        max-width: 44rem;
+    }
+</style>