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/api/badges/leaderboard/route.ts

@@ -3,6 +3,7 @@ import { apiJson } from "@/lib/api";
 import { bearerUserId } from "@/lib/api-auth";
 import { auth } from "@/lib/auth";
 import { prisma } from "@/lib/prisma";
+import { logger } from "@/lib/logger";
 
 export const dynamic = "force-dynamic";
 
@@ -314,6 +315,7 @@ export async function GET(req: Request) {
     ]);
 
     const rarity = Object.fromEntries(
+      // eslint-disable-next-line security/detect-object-injection -- rk from rarityKeys const, i is array index
       rarityKeys.map((rk, i) => [rk, rarityBoards[i]]),
     ) as Record<BadgeRarityKey, BadgeLeaderboardBoard>;
 
@@ -324,7 +326,7 @@ export async function GET(req: Request) {
       leaderboards: { totalBadges, achievementLevel, rarity },
     });
   } catch (err) {
-    console.error("Badge leaderboard error:", err);
+    logger.error("Badge leaderboard error", { module: "badges/leaderboard", error: String(err) });
     return apiJson({
       viewerUserId: 0,
       badgeStats: [],