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/videos/+page.svelte

@@ -200,7 +200,7 @@ const filteredVideos = $derived(() => {
   <!-- Videos 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 filteredVideos() as video}
+      {#each filteredVideos() as video (video.slug)}
         <Card
           class="p-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"
         >

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

@@ -33,7 +33,7 @@ let commentError = $state();
 let currentPlayId = $state<string | null>(null);
 let lastTrackedTime = $state(0);
 
-const relatedVideos = [
+const _relatedVideos = [
 	{
 		id: 2,
 		title: "Sunset Dreams",
@@ -94,7 +94,7 @@ async function handleLike() {
 	}
 }
 
-function handleBookmark() {
+function _handleBookmark() {
 	isBookmarked = !isBookmarked;
 }
 
@@ -279,7 +279,7 @@ let showPlayer = $state(false);
             />
             <!-- <Button
               variant={isBookmarked ? "default" : "outline"}
-              onclick={handleBookmark}
+              onclick={_handleBookmark}
               class="flex items-center gap-2 {isBookmarked
                 ? 'bg-gradient-to-r from-primary to-accent'
                 : 'border-primary/20 hover:bg-primary/10'}"
@@ -291,7 +291,7 @@ let showPlayer = $state(false);
 
           <!-- Model Info -->
           <div class="grid grid-cols-1 gap-4">
-            {#each data.video.models as model}
+            {#each data.video.models as model (model.slug)}
               <div class="flex items-center justify-between">
                 <div class="flex items-center gap-4">
                   <a href={`/models/${model.slug}`}>
@@ -341,7 +341,7 @@ let showPlayer = $state(false);
             <CardContent class="p-4">
               <p class="text-muted-foreground mb-4">{data.video.description}</p>
               <div class="flex flex-wrap gap-2">
-                {#each data.video.tags as tag}
+                {#each data.video.tags as tag (tag)}
                   <a
                     class="text-xs bg-primary/10 text-primary px-2 py-1 rounded-full"
                     href="/tags/{tag}"
@@ -438,7 +438,7 @@ let showPlayer = $state(false);
               {#if showComments}
                 <!-- Comments List -->
                 <div class="space-y-4">
-                  {#each data.comments as comment}
+                  {#each data.comments as comment (comment.id)}
                     <div class="flex gap-3">
                       <a href="/users/{comment.user_created.id}" class="flex-shrink-0">
                         <Avatar