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/+page.svelte

@@ -261,7 +261,7 @@ const filteredArticles = $derived(() => {
 
     <!-- Articles Grid -->
     <div class="grid grid-cols-1 md:grid-cols-2 lg:grid-cols-3 gap-8">
-      {#each filteredArticles() as article}
+      {#each filteredArticles() as article (article.slug)}
         <Card
           class="p-0 group hover:shadow-2xl hover:shadow-primary/25 transition-all duration-300 hover:-translate-y-3 bg-gradient-to-br from-card/90 via-card/95 to-card/85 backdrop-blur-xl shadow-lg shadow-primary/10 overflow-hidden"
         >
@@ -316,7 +316,7 @@ const filteredArticles = $derived(() => {
 
             <!-- Tags -->
             <div class="flex flex-wrap gap-2 mb-4">
-              {#each article.tags.slice(0, 3) as tag}
+              {#each article.tags.slice(0, 3) as tag (tag)}
                 <a
                   class="text-xs bg-primary/10 text-primary px-2 py-1 rounded-full"
                   href="/tags/{tag}"