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/news/[slug]/page.tsx

@@ -67,7 +67,7 @@ export default async function ArticlePage({
 
   const session = await auth();
   const loggedIn = Boolean(session?.user?.id);
-  const sessionUserId = loggedIn ? Number(session!.user!.id) : null;
+  const sessionUserId = session?.user?.id ? Number(session.user.id) : null;
 
   // Reaction counts grouped by reaction type for this article, plus the
   // signed-in user's currently-active reaction (so its button reads as pressed).

src/app/news/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 } from "@/components/public/ui";
 import { excerpt } from "@/lib/format";
 import { prisma } from "@/lib/prisma";
@@ -53,11 +54,15 @@ export default async function NewsPage() {
                 }}
               >
                 {a.image ? (
-                  <img
-                    src={a.image}
-                    alt=""
-                    style={{ width: "100%", aspectRatio: "16/9", objectFit: "cover" }}
-                  />
+                  <div style={{ position: "relative", width: "100%", aspectRatio: "16/9" }}>
+                    <Image
+                      src={a.image}
+                      alt=""
+                      fill
+                      style={{ objectFit: "cover" }}
+                      unoptimized
+                    />
+                  </div>
                 ) : (
                   <div
                     style={{ width: "100%", aspectRatio: "16/9", backgroundColor: "color-mix(in srgb, var(--color-primary) 10%, var(--color-navbar))" }}