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/lib/auth.ts

@@ -19,7 +19,9 @@ async function verify2faCode(userId: number, code: string): Promise<boolean> {
 
   // Try TOTP first
   try {
-    const secret = new LaravelEncrypter(env.APP_KEY!).decrypt(user.twoFactorSecret);
+    const appKey = env.APP_KEY;
+    if (!appKey) throw new Error("APP_KEY not configured");
+    const secret = new LaravelEncrypter(appKey).decrypt(user.twoFactorSecret);
     if (verifyTotp(code, secret)) return true;
   } catch { /* fall through to recovery */ }