chore: production readiness audit

- Move plans/ to dev-docs/, delete unused scaffold SVGs
- Create .dockerignore, rename container to immich-folio
- Extract BackLink component and toPhotoItems() helper in page.tsx
- Add keyboard accessibility (role=button, tabIndex, onKeyDown) to PhotoGrid
- Add aria-label to Lightbox EXIF toggle
- Fix React lint: replace setState-in-effect with useState initializer
- Add rate limiting to /api/auth (brute-force protection, 10 RPM)
- Add HSTS and X-DNS-Prefetch-Control security headers
- Add Secure flag to auth cookie in production
- New unit tests: rate-limit.test.ts (5), auth.test.ts (12)
- All 47 tests pass, 0 lint errors, build clean

components/BackLink.tsx

@@ -0,0 +1,29 @@
+/**
+ * BackLink — reusable "← Back to X" navigation link.
+ * Used in album headers to navigate back to a parent subpage or homepage.
+ */
+
+import Link from 'next/link';
+
+interface BackLinkProps {
+  href: string;
+  label: string;
+}
+
+export function BackLink({ href, label }: BackLinkProps) {
+  return (
+    <Link href={href} className="album-header__back">
+      <svg
+        viewBox="0 0 24 24"
+        fill="none"
+        stroke="currentColor"
+        strokeWidth="2"
+        strokeLinecap="round"
+        strokeLinejoin="round"
+      >
+        <polyline points="15 18 9 12 15 6" />
+      </svg>
+      {label}
+    </Link>
+  );
+}