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

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

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

@@ -0,0 +1,53 @@
+import type { ContentfulStatusCode } from 'hono/utils/http-status'
+import type { AppError } from '@/lib/errors'
+import type { ProblemDetails } from './problem-details'
+
+function statusFromError(error: AppError): ContentfulStatusCode {
+  switch (error.type) {
+    case 'validation':
+      return 400
+    case 'unauthorized':
+      return 401
+    case 'forbidden':
+      return 403
+    case 'not_found':
+      return 404
+    case 'conflict':
+      return 409
+    case 'unexpected':
+    default:
+      return 500
+  }
+}
+
+function titleFromStatus(status: ContentfulStatusCode): string {
+  switch (status) {
+    case 400:
+      return 'Bad Request'
+    case 401:
+      return 'Unauthorized'
+    case 403:
+      return 'Forbidden'
+    case 404:
+      return 'Not Found'
+    case 409:
+      return 'Conflict'
+    default:
+      return 'Internal Server Error'
+  }
+}
+
+export function mapAppErrorToProblem(
+  error: AppError,
+  instance?: string
+): ProblemDetails {
+  const status = statusFromError(error)
+
+  return {
+    type: `https://api.myapp.dev/problems/${error.type}`,
+    title: titleFromStatus(status),
+    status,
+    detail: status === 500 ? 'Internal server error' : error.message,
+    instance
+  }
+}