ci(github): add linting and improve CI workflow

- Add ESLint configuration for backend TypeScript code
- Include linting step in backend quality checks
- Add linting step to GitHub Actions CI workflow
- Enable configurable Prisma query logging via PRISMA_LOG_QUERIES environment variable
- Update PrismaService to support dynamic log levels based on PRISMA_LOG_QUERIES
- Replace BadRequestException with UnauthorizedException in receipt import security tests

backend/eslint.config.mjs

@@ -0,0 +1,26 @@
+import tseslint from '@typescript-eslint/eslint-plugin';
+import tsParser from '@typescript-eslint/parser';
+
+export default [
+  {
+    ignores: ['dist/**', 'node_modules/**'],
+  },
+  {
+    files: ['src/**/*.ts'],
+    languageOptions: {
+      parser: tsParser,
+      parserOptions: {
+        sourceType: 'module',
+      },
+    },
+    plugins: {
+      '@typescript-eslint': tseslint,
+    },
+    rules: {
+      '@typescript-eslint/no-unused-vars': [
+        'warn',
+        { argsIgnorePattern: '^_', varsIgnorePattern: '^_' },
+      ],
+    },
+  },
+];