feat: init payments + split payment in project workspace

- initProjectPayments: crea Acconto 50%/Saldo 50% se il progetto non ha pagamenti
- splitPayment: divide un pagamento in due rate con importi custom
- SplitPaymentForm: form client interattivo con anteprima live della rata 2
- PaymentsTab: stato vuoto con CTA + pulsante Dividi per ogni riga
- projectId prop opzionale per attivare le feature solo nel contesto progetto

src/app/admin/projects/[id]/page.tsx

@@ -86,6 +86,7 @@ export default async function ProjectDetailPage({
             payments={payments}
             acceptedTotal={project.accepted_total ?? "0"}
             clientId={id}
+            projectId={id}
           />
         </TabsContent>
 

src/app/admin/projects/project-actions.ts

@@ -4,7 +4,7 @@ import { revalidatePath } from "next/cache";
 import { getServerSession } from "next-auth";
 import { authOptions } from "@/lib/auth";
 import { db } from "@/db";
-import { projects, clients } from "@/db/schema";
+import { projects, clients, payments } from "@/db/schema";
 import { eq } from "drizzle-orm";
 import { nanoid } from "nanoid";
 
@@ -51,5 +51,60 @@ export async function unarchiveProject(projectId: string): Promise<void> {
 export async function updateProjectAcceptedTotal(projectId: string, acceptedTotal: string): Promise<void> {
   await requireAdmin();
   await db.update(projects).set({ accepted_total: acceptedTotal }).where(eq(projects.id, projectId));
+  revalidatePath(`/admin/projects/${projectId}`);
+}
+
+// Creates Acconto 50% + Saldo 50% stubs for a project that has no payments yet.
+export async function initProjectPayments(projectId: string): Promise<void> {
+  await requireAdmin();
+  const rows = await db
+    .select({ accepted_total: projects.accepted_total })
+    .from(projects)
+    .where(eq(projects.id, projectId))
+    .limit(1);
+  if (!rows[0]) throw new Error("Progetto non trovato");
+  const total = parseFloat(rows[0].accepted_total ?? "0");
+  const half = (total / 2).toFixed(2);
+  await db.insert(payments).values([
+    { project_id: projectId, label: "Acconto 50%", amount: half, status: "da_saldare" },
+    { project_id: projectId, label: "Saldo 50%", amount: half, status: "da_saldare" },
+  ]);
+  revalidatePath(`/admin/projects/${projectId}`);
+}
+
+// Splits one payment into two with custom amounts. The original row keeps
+// its label (+ " – Rata 1"), a new row is created with the remainder.
+export async function splitPayment(
+  paymentId: string,
+  projectId: string,
+  firstAmountStr: string
+): Promise<void> {
+  await requireAdmin();
+  const first = parseFloat(firstAmountStr);
+  if (isNaN(first) || first <= 0) throw new Error("Importo non valido");
+
+  const rows = await db.select().from(payments).where(eq(payments.id, paymentId)).limit(1);
+  if (!rows[0]) throw new Error("Pagamento non trovato");
+
+  const original = rows[0];
+  const total = parseFloat(original.amount);
+  const second = parseFloat((total - first).toFixed(2));
+  if (second <= 0) throw new Error("Il primo importo deve essere inferiore al totale");
+
+  // Strip any existing " – Rata N" suffix so re-splits stay clean.
+  const baseLabel = original.label.replace(/ – Rata \d+$/, "");
+
+  await db
+    .update(payments)
+    .set({ amount: first.toFixed(2), label: `${baseLabel} – Rata 1` })
+    .where(eq(payments.id, paymentId));
+
+  await db.insert(payments).values({
+    project_id: projectId,
+    label: `${baseLabel} – Rata 2`,
+    amount: second.toFixed(2),
+    status: original.status,
+  });
+
   revalidatePath(`/admin/projects/${projectId}`);
 }