Security hardening, code quality, and ESLint setup

- Remove production DB dump (db_backup_*.sql) and update.log from git tracking
- Add DB backups to .gitignore
- Replace all console.log/console.error with structured logger module
- Translate Dutch error messages to English (link-discord.ts)
- Remove dead code blocks (register-form.tsx false && pattern)
- Add ESLint flat config with TypeScript, React, Next.js, jsx-a11y, and security plugins
- Add Prettier config
- Add eslint-plugin-security for security-aware linting
- Fix all 119+ ESLint warnings across the codebase:
  - Resolve security/detect-object-injection with safe access patterns
  - Resolve security/detect-non-literal-fs-filename with path traversal validation
  - Replace <img> with next/image <Image> component
  - Remove unused variables and imports
  - Replace non-null assertions with proper type guards
  - Replace <a> with <Link> for internal navigation
  - Use next/script Script component for external scripts
- Fix setState-in-useEffect anti-patterns (navbar-color-picker, logo-generator, theme-switcher)
- Add lint and format scripts to package.json

All checks: typecheck ✓, tests 58/58 ✓, lint 0 errors 0 warnings ✓

src/app/admin/radio/autodj/page.tsx

@@ -1,4 +1,5 @@
 import Link from 'next/link';
+import Image from 'next/image';
 import { createTrack, deleteTrack, toggleTrack } from '@/actions/admin-radio-autodj';
 import { StatusCard } from '@/components/admin/dashboard';
 import { prisma } from '@/lib/prisma';
@@ -220,12 +221,13 @@ export default async function AdminRadioAutoDjPage() {
                     <td>
                       <span className="inline-flex gap-2 items-center">
                         {track.artworkUrl ? (
-                          <img
+                          <Image
                             src={track.artworkUrl}
                             alt=""
                             width={32}
                             height={32}
                             style={{ borderRadius: 4, objectFit: 'cover' }}
+                            unoptimized
                           />
                         ) : null}
                         <strong>{track.title}</strong>