chore(13-01): install ESLint+Prettier, author flat config, wire lint/format scripts

- Install eslint@9.39.4 @eslint/js@9.39.4 typescript-eslint@8.61.0
  eslint-plugin-react@7.37.5 eslint-plugin-react-hooks@7.1.1
  eslint-config-prettier@10.1.8 prettier@3.8.4 as root devDependencies
- Author eslint.config.js: ignores → base recommendedTypeChecked (projectService:true)
  → pwa-react block (apps/pwa/** only) → disableTypeChecked override (config files +
  apps/api/tests) → prettier-last (eslint-config-prettier/flat)
- Add .prettierrc (singleQuote:true, printWidth:100) and .prettierignore
- Add "type":"module" to root package.json; add format/format:check scripts
- Add lint script (--max-warnings 0) to apps/api and apps/pwa

eslint.config.js

@@ -0,0 +1,88 @@
+// eslint.config.js — root flat ESLint config (ESM)
+// Covers: apps/api (Node/TS) + apps/pwa (React 19 + TypeScript)
+// typescript-eslint v8, projectService: true (auto-discovers all tsconfigs)
+// ESLint pinned to 9.39.4 — do NOT upgrade to ESLint 10 until eslint-plugin-react
+// resolves the "getFilename is not a function" incompatibility (jsx-eslint#3977).
+
+import js from '@eslint/js'
+import tseslint from 'typescript-eslint'
+import reactPlugin from 'eslint-plugin-react'
+import reactHooks from 'eslint-plugin-react-hooks'
+import prettierConfig from 'eslint-config-prettier/flat'
+
+export default tseslint.config(
+  // ── 1. Global ignores (replaces .eslintignore) ────────────────────────────
+  {
+    ignores: [
+      '**/dist/**',           // build output: apps/api/dist, apps/pwa/dist, generated sw.js
+      '**/node_modules/**',
+      'apps/api/src/db/migrations/**', // generated Drizzle SQL files
+      'pnpm-lock.yaml',
+    ],
+  },
+
+  // ── 2. Base: all TS/TSX files in both apps ────────────────────────────────
+  // Uses recommendedTypeChecked (type-aware) per D-13-01.
+  // NOT strict/strictTypeChecked — D-13-03 explicitly rejects the extra churn.
+  {
+    files: ['apps/**/*.{ts,tsx}'],
+    extends: [
+      js.configs.recommended,
+      tseslint.configs.recommendedTypeChecked,
+    ],
+    languageOptions: {
+      parserOptions: {
+        projectService: true,
+        tsconfigRootDir: import.meta.dirname,
+      },
+    },
+    rules: {
+      // Allow _-prefixed names as intentionally unused (params + vars)
+      '@typescript-eslint/no-unused-vars': [
+        'error',
+        { argsIgnorePattern: '^_', varsIgnorePattern: '^_' },
+      ],
+    },
+  },
+
+  // ── 3. React + Hooks: PWA only (D-13-02) ────────────────────────────────
+  // Apps/api is Node/TS only — React config does NOT apply there.
+  {
+    files: ['apps/pwa/**/*.{ts,tsx}'],
+    extends: [
+      reactPlugin.configs.flat.recommended,
+      reactHooks.configs.flat.recommended,
+    ],
+    settings: {
+      react: { version: 'detect' },
+    },
+    rules: {
+      // TypeScript props typing supersedes prop-types in React 19 (RESEARCH Open Question 2)
+      'react/prop-types': 'off',
+    },
+  },
+
+  // ── 4. disableTypeChecked override (D-13-10) ─────────────────────────────
+  // These files are outside all tsconfig projects (tool configs + API test dir).
+  // projectService: true cannot type-check them → disable type-aware rules only.
+  // Non-type-aware rules (syntax, no-unused-vars) still apply.
+  // Source: typescript-eslint.io/troubleshooting/typed-linting/#i-get-errors-telling-me-the-file-must-be-included
+  {
+    files: [
+      'apps/api/drizzle.config.ts',
+      'apps/api/vitest.config.ts',
+      'apps/pwa/vite.config.ts',
+      'apps/pwa/vitest.config.ts',
+      'apps/pwa/playwright.config.ts',
+      'apps/api/tests/**/*.ts', // excluded from apps/api/tsconfig.json (Pitfall 2)
+      'eslint.config.js',       // this file itself (not a ts project member)
+    ],
+    extends: [tseslint.configs.disableTypeChecked],
+  },
+
+  // ── 5. eslint-config-prettier: MUST BE LAST ──────────────────────────────
+  // Disables all ESLint formatting rules that conflict with Prettier (D-13-07).
+  // Use the /flat import path for ESM flat config (Pitfall 7).
+  // Source: github.com/prettier/eslint-config-prettier
+  prettierConfig,
+)