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/rankings/page.tsx

@@ -1,5 +1,6 @@
 import { getTranslations } from "next-intl/server";
 import Link from "next/link";
+import Image from "next/image";
 import { ContentCard, EmptyState, OnlineBadge, RankBadge } from "@/components/public/ui";
 import { avatarImageUrl } from "@/lib/format";
 import { prisma } from "@/lib/prisma";
@@ -53,13 +54,13 @@ export default async function RankingsPage() {
                   style={{ display: "flex", gap: "0.9rem", alignItems: "center" }}
                 >
                   <RankBadge position={i + 1} />
-                  {/* biome-ignore lint/performance/noImgElement: external avatar imager */}
-                  <img
+                  <Image
                     className="avatar"
                     src={avatar}
                     alt={`${u.username} avatar`}
                     width={50}
                     height={90}
+                    unoptimized
                   />
                   <div style={{ minWidth: 0, flex: 1 }}>
                     <h3 style={{ margin: "0 0 0.25rem", fontSize: "1rem" }}>