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>

package.json

@@ -2,14 +2,19 @@
 	"name": "sexy.pivoine.art",
 	"version": "1.0.0",
 	"description": "",
-	"main": "index.js",
+	"type": "module",
 	"scripts": {
 		"test": "echo \"Error: no test specified\" && exit 1",
 		"build:frontend": "git pull && pnpm install && pnpm --filter @sexy.pivoine.art/frontend build",
 		"build:backend": "git pull && pnpm install && pnpm --filter @sexy.pivoine.art/backend build",
 		"dev:data": "docker compose up -d postgres redis",
 		"dev:backend": "pnpm --filter @sexy.pivoine.art/backend dev",
-		"dev": "pnpm dev:data && pnpm dev:backend & pnpm --filter @sexy.pivoine.art/frontend dev"
+		"dev": "pnpm dev:data && pnpm dev:backend & pnpm --filter @sexy.pivoine.art/frontend dev",
+		"lint": "eslint .",
+		"lint:fix": "eslint . --fix",
+		"format": "prettier --write .",
+		"format:check": "prettier --check .",
+		"check": "pnpm -r --filter=!sexy.pivoine.art check"
 	},
 	"keywords": [],
 	"author": {
@@ -30,5 +35,14 @@
 			"@tailwindcss/oxide",
 			"node-sass"
 		]
+	},
+	"devDependencies": {
+		"@eslint/js": "^10.0.1",
+		"eslint": "^10.0.2",
+		"eslint-config-prettier": "^10.1.8",
+		"eslint-plugin-svelte": "^3.15.0",
+		"globals": "^17.4.0",
+		"prettier": "^3.8.1",
+		"typescript-eslint": "^8.56.1"
 	}
 }