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/+page.svelte

@@ -76,7 +76,7 @@ const { data } = $props();
     </div>
 
     <div class="grid grid-cols-1 md:grid-cols-3 gap-8 max-w-3xl mx-auto">
-      {#each data.models as model}
+      {#each data.models as model (model.slug)}
         <Card
           class="p-0 group hover:shadow-2xl hover:shadow-primary/20 transition-all duration-300 hover:-translate-y-2 bg-gradient-to-br from-card to-card/50 border-primary/20"
         >
@@ -128,7 +128,7 @@ const { data } = $props();
     </div>
 
     <div class="grid grid-cols-1 md:grid-cols-2 gap-8 max-w-6xl mx-auto">
-      {#each data.videos as video}
+      {#each data.videos as video (video.slug)}
         <Card
           class="p-0 group hover:shadow-2xl hover:shadow-accent/20 transition-all duration-300 hover:-translate-y-2 bg-gradient-to-br from-card to-card/50 border-accent/20 overflow-hidden"
         >