fix(numbering): preview the next FREE offer number, not a stale one

previewOfferNumber returned applyPattern(previewNextSequence) directly; when the number_sequences 'offer' counter lags behind real quotations, that raw next number is already taken, so the offer editor showed an already-used number (e.g. NA-2026-0001 when 0001/0002 exist). Mirror generateOfferNumber's collision loop in the preview (without consuming the sequence). +2 tests.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/__tests__/numbering.test.ts

@@ -4,6 +4,8 @@ import {
   generateOfferNumber,
   previewSharedNumber,
   isSharedNumberTaken,
+  previewOfferNumber,
+  isOfferNumberTaken,
 } from "../services/numbering.service";
 import prisma from "../config/database";
 
@@ -90,3 +92,43 @@ describe("previewSharedNumber", () => {
     expect(await isSharedNumberTaken(second)).toBe(false);
   });
 });
+
+describe("previewOfferNumber", () => {
+  let createdQuotationId: number | null = null;
+
+  beforeEach(async () => {
+    await prisma.number_sequences.deleteMany({ where: { type: "offer" } });
+  });
+
+  afterEach(async () => {
+    if (createdQuotationId !== null) {
+      await prisma.quotations
+        .delete({ where: { id: createdQuotationId } })
+        .catch(() => {});
+      createdQuotationId = null;
+    }
+    await prisma.number_sequences.deleteMany({ where: { type: "offer" } });
+  });
+
+  it("returns a number not already used by a quotation", async () => {
+    const preview = await previewOfferNumber();
+    expect(typeof preview).toBe("string");
+    expect(await isOfferNumberTaken(preview)).toBe(false);
+  });
+
+  it("skips a number already taken when the sequence counter lags behind", async () => {
+    // Fresh counter → preview points at the first sequence number; with the
+    // skip-taken logic it's guaranteed free, so we can claim it.
+    const first = await previewOfferNumber();
+    const quotation = await prisma.quotations.create({
+      data: { quotation_number: first },
+    });
+    createdQuotationId = quotation.id;
+
+    // The counter still lags, but `first` is now taken — the preview must
+    // advance past it instead of re-offering it.
+    const second = await previewOfferNumber();
+    expect(second).not.toBe(first);
+    expect(await isOfferNumberTaken(second)).toBe(false);
+  });
+});