feat: add Zod validation for auth endpoints

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/schemas/common.ts

@@ -0,0 +1,12 @@
+import { ZodSchema, ZodError } from 'zod';
+
+export function parseBody<T>(schema: ZodSchema<T>, body: unknown): { data: T } | { error: string } {
+  try {
+    return { data: schema.parse(body) };
+  } catch (e) {
+    if (e instanceof ZodError) {
+      return { error: e.errors.map(err => err.message).join(', ') };
+    }
+    return { error: 'Neplatný požadavek' };
+  }
+}