feat: implement error handling and validation middleware, enhance result handling

Co-authored-by: Copilot <copilot@github.com>

apps/backend/src/lib/http/problem-builders.ts

@@ -0,0 +1,29 @@
+// http/problem-builders.ts
+import type { ProblemDetails } from './problem-details'
+
+export function validationProblem(params: {
+  detail?: string
+  instance?: string
+  errors?: Record<string, string[]>
+}): ProblemDetails {
+  return {
+    type: 'https://api.myapp.dev/problems/validation',
+    title: 'Bad Request',
+    status: 400,
+    detail: params.detail ?? 'Invalid request data',
+    instance: params.instance,
+    errors: params.errors
+  }
+}
+
+export function unexpectedProblem(params?: {
+  instance?: string
+}): ProblemDetails {
+  return {
+    type: 'https://api.myapp.dev/problems/unexpected',
+    title: 'Internal Server Error',
+    status: 500,
+    detail: 'Internal server error',
+    instance: params?.instance
+  }
+}