feat(projects): add createProject service (manual, shared numbering)

Introduces CreateProjectSchema (Zod 4, NaN-guarded coercions), the
createProject() service function (atomic shared-number generation inside
a $transaction), getNextProjectNumber() preview helper, and two Vitest
tests that verify standalone project creation and distinct sequential
numbers. Routes are deferred to a subsequent task.

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

src/schemas/projects.schema.ts

@@ -1,5 +1,27 @@
 import { z } from "zod";
 
+export const CreateProjectSchema = z.object({
+  name: z.string().min(1, "Zadejte název projektu"),
+  customer_id: z
+    .union([z.number(), z.string(), z.null()])
+    .transform((v) => (v === null || v === "" ? null : Number(v)))
+    .refine((v) => v === null || !Number.isNaN(v), {
+      message: "Neplatný zákazník",
+    })
+    .optional(),
+  responsible_user_id: z
+    .union([z.number(), z.string(), z.null()])
+    .transform((v) => (v === null || v === "" ? null : Number(v)))
+    .refine((v) => v === null || !Number.isNaN(v), {
+      message: "Neplatná zodpovědná osoba",
+    })
+    .optional(),
+  status: z.string().optional().default("aktivni"),
+  start_date: z.union([z.string(), z.null()]).optional(),
+  end_date: z.union([z.string(), z.null()]).optional(),
+  notes: z.string().nullish(),
+});
+
 export const UpdateProjectSchema = z.object({
   name: z.string().nullish(),
   status: z.string().optional(),
@@ -28,5 +50,6 @@ export const CreateProjectNoteSchema = z.object({
   content: z.string().nullish(),
 });
 
+export type CreateProjectInput = z.infer<typeof CreateProjectSchema>;
 export type UpdateProjectInput = z.infer<typeof UpdateProjectSchema>;
 export type CreateProjectNoteInput = z.infer<typeof CreateProjectNoteSchema>;