feat: add formidable ESLint + Prettier linting setup

- Root-level eslint.config.js (flat config): typescript-eslint,
  eslint-plugin-svelte, eslint-config-prettier, @eslint/js
- Root-level prettier.config.js with prettier-plugin-svelte
- svelte-check added to frontend for Svelte/TS type checking
- lint, lint:fix, format, format:check, check scripts in root
  and both packages
- All 60 lint errors fixed across backend and frontend:
  - Consistent type imports
  - Removed unused imports/variables
  - Added keys to all {#each} blocks for Svelte performance
  - Replaced mutable Set/Map with SvelteSet/SvelteMap
  - Fixed useless assignments and empty catch blocks
- 64 remaining warnings are intentional any usages in the
  Pothos/Drizzle GraphQL resolver layer

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/frontend/src/routes/magazine/[slug]/+page.svelte

@@ -6,7 +6,6 @@ import { Card, CardContent } from "$lib/components/ui/card";
 import { calcReadingTime } from "$lib/utils";
 import TimeAgo from "javascript-time-ago";
 import { getAssetUrl } from "$lib/directus";
-import SharingPopup from "$lib/components/sharing-popup/sharing-popup.svelte";
 import Meta from "$lib/components/meta/meta.svelte";
 import PeonyBackground from "$lib/components/background/peony-background.svelte";
 import SharingPopupButton from "$lib/components/sharing-popup/sharing-popup-button.svelte";
@@ -129,7 +128,7 @@ const timeAgo = new TimeAgo("en");
                         <span class="font-semibold">Tags</span>
                     </div>
                     <div class="flex flex-wrap gap-2">
-                        {#each data.article.tags as tag}
+                        {#each data.article.tags as tag (tag)}
                             <a class="bg-primary/10 text-primary px-3 py-1 rounded-full text-sm" href="/tags/{tag}">
                                 #{tag}
                             </a>