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/lib/components/recording-card/recording-card.svelte

@@ -94,7 +94,7 @@ function getStatusColor(status: string): string {
 
 		<!-- Device Info -->
 		<div class="space-y-1">
-			{#each recording.device_info.slice(0, 2) as device}
+			{#each recording.device_info.slice(0, 2) as device (device.name)}
 				<div
 					class="flex items-center gap-2 text-xs text-muted-foreground bg-muted/20 rounded px-2 py-1"
 				>
@@ -117,7 +117,7 @@ function getStatusColor(status: string): string {
 		<!-- Tags -->
 		{#if recording.tags && recording.tags.length > 0}
 			<div class="flex flex-wrap gap-1">
-				{#each recording.tags as tag}
+				{#each recording.tags as tag (tag)}
 					<span
 						class="text-xs px-2 py-0.5 rounded-full bg-primary/10 text-primary border border-primary/20"
 					>