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/backend/src/lib/gamification.ts

@@ -74,7 +74,7 @@ export async function updateUserStats(db: DB, userId: string): Promise<void> {
     .where(eq(recordings.user_id, userId));
   const ownIds = ownRecordingIds.map((r) => r.id);
 
-  let playbacksCount = 0;
+  let playbacksCount: number;
   if (ownIds.length > 0) {
     const playbacksResult = await db.execute(sql`
       SELECT COUNT(*) as count FROM recording_plays