feat(web/m7-381): admin overview Library Scan section + manual trigger

Adds getScanStatus/triggerScan helpers, createScanStatusQuery factory
(3s poll), qk.scanStatus(), and a Library Scan section on the admin
overview page with per-stage tallies and inline Run scan button.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/src/routes/admin/+page.svelte

@@ -6,12 +6,14 @@
     createAdminRequestsQuery,
     createLidarrConfigQuery,
     createAdminQuarantineQuery,
+    createScanStatusQuery,
     approveRequest,
     rejectRequest,
     resolveQuarantine,
     deleteQuarantineFile,
     deleteQuarantineViaLidarr,
-    refetchMissingCovers
+    refetchMissingCovers,
+    triggerScan
   } from '$lib/api/admin';
   import { qk } from '$lib/api/queries';
   import { copyForCode } from '$lib/api/error-copy';
@@ -178,6 +180,41 @@
     }
   }
 
+  // ---- Library scan ----
+  const scanStatusStore = $derived(createScanStatusQuery());
+  const scanStatusQ = $derived($scanStatusStore);
+  const scan = $derived(scanStatusQ.data);
+  const scanInFlight = $derived(scan?.in_flight === true);
+
+  let triggering = $state(false);
+  let triggerResult = $state<string | null>(null);
+
+  async function onTriggerScan() {
+    if (triggering || scanInFlight) return;
+    triggering = true;
+    triggerResult = null;
+    try {
+      await triggerScan();
+      triggerResult = 'Scan started.';
+      await client.invalidateQueries({ queryKey: qk.scanStatus() });
+    } catch (e) {
+      const code = (e as { code?: string; status?: number })?.code ?? 'unknown';
+      const status = (e as { status?: number })?.status;
+      if (status === 409) {
+        triggerResult = 'A scan is already running.';
+      } else {
+        triggerResult = `Failed: ${code}`;
+      }
+    } finally {
+      triggering = false;
+    }
+  }
+
+  function formatTime(iso: string | null | undefined): string {
+    if (!iso) return '—';
+    return new Date(iso).toLocaleString();
+  }
+
   // ---- Cover art bulk refetch ----
   let bulkBusy = $state(false);
   let bulkResult = $state<string | null>(null);
@@ -276,6 +313,86 @@
     {/if}
   </section>
 
+  <!-- Library scan -->
+  <section class="rounded border border-border bg-surface p-4">
+    <div class="flex items-start justify-between gap-4">
+      <div>
+        <h2 class="font-display text-lg font-medium">Library scan</h2>
+        <p class="mt-1 text-sm text-text-secondary">
+          Walks the music tree, fills missing MusicBrainz IDs from tags, and refetches album covers.
+        </p>
+      </div>
+      <button
+        type="button"
+        onclick={onTriggerScan}
+        disabled={triggering || scanInFlight}
+        class="rounded-md bg-action-primary px-4 py-2 text-sm font-medium text-action-fg hover:opacity-90 disabled:opacity-50"
+      >
+        {scanInFlight ? 'Scanning…' : triggering ? 'Starting…' : 'Run scan'}
+      </button>
+    </div>
+
+    {#if !scan || !scan.id}
+      <p class="mt-3 text-sm text-text-muted">No scan has run yet.</p>
+    {:else}
+      <div class="mt-3 grid gap-3 sm:grid-cols-3">
+        <!-- Library stage -->
+        <div class="rounded border border-border bg-bg p-3">
+          <div class="text-xs font-medium uppercase tracking-wide text-text-muted">Library walk</div>
+          {#if scan.library}
+            <dl class="mt-2 space-y-0.5 text-sm">
+              <div class="flex justify-between"><dt class="text-text-secondary">Scanned</dt><dd>{scan.library.scanned}</dd></div>
+              <div class="flex justify-between"><dt class="text-text-secondary">Added</dt><dd>{scan.library.added}</dd></div>
+              <div class="flex justify-between"><dt class="text-text-secondary">Updated</dt><dd>{scan.library.updated}</dd></div>
+              <div class="flex justify-between"><dt class="text-text-secondary">Skipped</dt><dd>{scan.library.skipped}</dd></div>
+              <div class="flex justify-between"><dt class="text-text-secondary">Errored</dt><dd>{scan.library.errored}</dd></div>
+            </dl>
+          {:else}
+            <p class="mt-2 text-sm text-text-muted">{scanInFlight ? 'Pending…' : 'Skipped.'}</p>
+          {/if}
+        </div>
+
+        <!-- MBID backfill stage -->
+        <div class="rounded border border-border bg-bg p-3">
+          <div class="text-xs font-medium uppercase tracking-wide text-text-muted">MBID backfill</div>
+          {#if scan.mbid_backfill}
+            <dl class="mt-2 space-y-0.5 text-sm">
+              <div class="flex justify-between"><dt class="text-text-secondary">Processed</dt><dd>{scan.mbid_backfill.processed}</dd></div>
+              <div class="flex justify-between"><dt class="text-text-secondary">Healed</dt><dd>{scan.mbid_backfill.healed}</dd></div>
+              <div class="flex justify-between"><dt class="text-text-secondary">Skipped</dt><dd>{scan.mbid_backfill.skipped}</dd></div>
+            </dl>
+          {:else}
+            <p class="mt-2 text-sm text-text-muted">{scanInFlight ? 'Pending…' : 'Skipped.'}</p>
+          {/if}
+        </div>
+
+        <!-- Cover enrichment stage -->
+        <div class="rounded border border-border bg-bg p-3">
+          <div class="text-xs font-medium uppercase tracking-wide text-text-muted">Cover enrichment</div>
+          {#if scan.cover_enrich}
+            <dl class="mt-2 space-y-0.5 text-sm">
+              <div class="flex justify-between"><dt class="text-text-secondary">Processed</dt><dd>{scan.cover_enrich.processed}</dd></div>
+              <div class="flex justify-between"><dt class="text-text-secondary">Succeeded</dt><dd>{scan.cover_enrich.succeeded}</dd></div>
+              <div class="flex justify-between"><dt class="text-text-secondary">Failed</dt><dd>{scan.cover_enrich.failed}</dd></div>
+            </dl>
+          {:else}
+            <p class="mt-2 text-sm text-text-muted">{scanInFlight ? 'Pending…' : 'Skipped.'}</p>
+          {/if}
+        </div>
+      </div>
+
+      <div class="mt-3 text-xs text-text-muted">
+        Started {formatTime(scan.started_at)}
+        {#if scan.finished_at}· Finished {formatTime(scan.finished_at)}{/if}
+        {#if scan.error_message}· <span class="text-oxblood">Error: {scan.error_message}</span>{/if}
+      </div>
+    {/if}
+
+    {#if triggerResult}
+      <p class="mt-2 text-sm">{triggerResult}</p>
+    {/if}
+  </section>
+
   <!-- Cover art bulk refetch -->
   <section class="rounded border border-border bg-surface p-4">
     <h2 class="font-display text-lg font-medium">Cover art</h2>