implement SkRateLimiterService with Leaky Bucket rate limiting

packages/backend/eslint.config.js

@@ -42,6 +42,13 @@ export default [
 				name: '__filename',
 				message: 'Not in ESModule. Use `import.meta.url` instead.',
 			}],
+			// https://typescript-eslint.io/rules/prefer-nullish-coalescing/
+			'@typescript-eslint/prefer-nullish-coalescing': ['warn', {
+				ignorePrimitives: {
+					// Without this, the rule breaks for nullable booleans
+					boolean: true,
+				},
+			}],
 		},
 	},
 	{