feat(ai): chat / extract-invoices / usage / budget routes

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/schemas/ai.schema.ts

@@ -0,0 +1,20 @@
+import { z } from "zod";
+
+export const AiChatSchema = z.object({
+  messages: z
+    .array(
+      z.object({
+        role: z.enum(["user", "assistant"]),
+        content: z.string().min(1).max(8000),
+      }),
+    )
+    .min(1, "Zpráva je povinná")
+    .max(100, "Příliš mnoho zpráv"),
+});
+
+export const AiBudgetSchema = z.object({
+  budget_usd: z
+    .union([z.number(), z.string()])
+    .transform((v) => Number(v))
+    .refine((n) => Number.isFinite(n) && n >= 0, "Neplatný rozpočet"),
+});