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/rate-limit.ts

@@ -32,7 +32,9 @@ function cleanup(): void {
     if (buckets.size > MAX_BUCKETS) {
       const sorted = [...buckets.entries()].sort((a, b) => a[1].resetAt - b[1].resetAt);
       const toRemove = Math.floor(sorted.length * 0.2);
-      for (let i = 0; i < toRemove; i++) buckets.delete(sorted[i][0]);
+      for (let i = 0; i < toRemove; i++)
+        // eslint-disable-next-line security/detect-object-injection -- numeric array index
+        buckets.delete(sorted[i][0]);
     }
   }
 }
@@ -47,7 +49,6 @@ export async function rateLimit(
   if (redis) {
     try {
       const windowKey = `ratelimit:${key}`;
-      const windowSec = Math.ceil(windowMs / 1000);
       const current = await redis.incr(windowKey);
       if (current === 1) await redis.pexpire(windowKey, windowMs);
       const ttl = current === 1 ? windowMs : Math.max(0, await redis.pttl(windowKey));