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

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

apps/backend/src/lib/http/zod-issues.ts

@@ -0,0 +1,18 @@
+type IssueLike = {
+  path: PropertyKey[]
+  message: string
+}
+
+export function zodIssuesToRecord(
+  issues: IssueLike[]
+): Record<string, string[]> {
+  const out: Record<string, string[]> = {}
+
+  for (const issue of issues) {
+    const key = issue.path.length ? issue.path.join('.') : 'root'
+    out[key] ??= []
+    out[key].push(issue.message)
+  }
+
+  return out
+}