feat: directory scanner plugin — walk library paths, auto-register assets

- DirectoryScannerPlugin: recursive directory walk via FileStoragePort
- Computes SHA256 checksums, classifies media by extension
- Registers each file via RegisterAssetHandler (triggers AssetIngested → extract_metadata pipeline)
- Reads library_path_id from job payload, looks up volume + path
- Seeded plugin + scan_directory pipeline
- Trigger via POST /jobs with { job_type: "ScanDirectory", payload: { library_path_id: "..." } }

k-photos-frontend/app/page.tsx

@@ -0,0 +1,19 @@
+import { Button } from "@/components/ui/button"
+
+export default function Page() {
+  return (
+    <div className="flex min-h-svh p-6">
+      <div className="flex max-w-md min-w-0 flex-col gap-4 text-sm leading-loose">
+        <div>
+          <h1 className="font-medium">Project ready!</h1>
+          <p>You may now add components and start building.</p>
+          <p>We&apos;ve already added the button component for you.</p>
+          <Button className="mt-2">Button</Button>
+        </div>
+        <div className="font-mono text-xs text-muted-foreground">
+          (Press <kbd>d</kbd> to toggle dark mode)
+        </div>
+      </div>
+    </div>
+  )
+}