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/models/+page.svelte

@@ -144,7 +144,7 @@ const filteredModels = $derived(() => {
     <!-- Models Grid -->
     <div class="container mx-auto px-4 py-12">
         <div class="grid grid-cols-1 md:grid-cols-2 lg:grid-cols-3 gap-8">
-            {#each filteredModels() as model}
+            {#each filteredModels() as model (model.slug)}
                 <Card
                     class="py-0 group hover:shadow-2xl hover:shadow-primary/25 transition-all duration-500 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"
                 >
@@ -211,7 +211,7 @@ const filteredModels = $derived(() => {
 
                         <!-- Tags -->
                         <div class="flex flex-wrap gap-2 mb-4">
-                            {#each model.tags as tag}
+                            {#each model.tags as tag (tag)}
                                 <a
                                     class="text-xs bg-primary/10 text-primary px-2 py-1 rounded-full"
                                     href="/tags/{tag}"

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

@@ -131,7 +131,7 @@ let totalPlays = $derived(
 
               <!-- Tags -->
               <div class="flex flex-wrap gap-2">
-                {#each data.model.tags as tag}
+                {#each data.model.tags as tag (tag)}
                   <a
                     class="text-xs bg-primary/10 text-primary px-3 py-1 rounded-full"
                     href="/tags/{tag}"
@@ -227,7 +227,7 @@ let totalPlays = $derived(
 
       <TabsContent value="videos">
         <div class="grid grid-cols-1 md:grid-cols-2 lg:grid-cols-3 gap-6">
-          {#each data.videos as video}
+          {#each data.videos as video (video.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 overflow-hidden"
             >