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

next.config.ts

@@ -33,6 +33,11 @@ const nextConfig: NextConfig = {
             key: 'Permissions-Policy',
             value: 'camera=(), microphone=(), geolocation=()',
           },
+          {
+            key: 'Strict-Transport-Security',
+            value: 'max-age=31536000; includeSubDomains',
+          },
+          { key: 'X-DNS-Prefetch-Control', value: 'off' },
         ],
       },
     ];