Address the 11 October test report findings

The initial test report (run before the pilot-readiness changes) raised
seven findings and a retest plan. This closes the ones still open.

- F-01: a business on its free trial may run 2 active branches whatever
  its tier, so an owner can try transfers and per-branch stock. Only
  active branches count towards any limit.
- F-06: owners can rename branches and deactivate/reactivate them from
  the Branches page. A branch can only be deactivated once it holds no
  stock and no active staff; its history stays in every report, and it
  leaves every picker.
- F-04: the Products page lists deactivated products under a toggle with
  Reactivate. Deactivating warns about stock still on hand, which stays
  on the Stock page so it can be transferred or counted out.
- F-05: products carry a unit (L, ml, kg, g, pcs, tray, pack) instead of
  everything being litres.
- CAT-12: product prices must be above zero and names are unique per
  business, ignoring case.
- F-03: a reversed report period is refused by the page and the API, and
  the page labels which period the cards show. Report dates are Nairobi
  business days.
- F-07: cost of goods sold is units sold x average buying price. Stock
  bought in the period is reported separately, with a cash surplus line,
  so unsold stock no longer shows as a loss.
- STA-04: staff and new branch managers can be invited with a one-time,
  7-day link; the person sets their own password. Managers can issue a
  new link for their own cashiers.
- OFF-02 / POS-02: the till shows each product's stock and deducts every
  sale immediately, online or offline; it warns before selling more than
  the recorded stock, and has a Clear sale button.
- npm run backfill:stock turns purchases recorded before the stock ledger
  into stock, so existing tenants (and the report's ZERRO fixtures) start
  from what they bought.

tests/retest-plan.test.ts runs the report's completion and retest plan
and checks every balance and total it predicts.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/lib/server/invites.ts

@@ -0,0 +1,27 @@
+import crypto from "crypto";
+import bcrypt from "bcryptjs";
+import type { NextRequest } from "next/server";
+import type { IUser } from "@/lib/server/models/User";
+
+export const INVITE_TTL_MS = 7 * 24 * 60 * 60 * 1000;
+
+export function hashInviteToken(token: string): string {
+  return crypto.createHash("sha256").update(token).digest("hex");
+}
+
+/** A password nobody knows, for an account whose owner has not accepted their invite yet. */
+export async function unusablePasswordHash(): Promise<string> {
+  return bcrypt.hash(crypto.randomBytes(32).toString("hex"), 12);
+}
+
+/**
+ * Give the user a fresh invite (any earlier link stops working) and return
+ * the link to send them. The caller saves the user.
+ */
+export function issueInvite(user: IUser, req: NextRequest): string {
+  const token = crypto.randomBytes(32).toString("base64url");
+  user.inviteTokenHash = hashInviteToken(token);
+  user.inviteExpiresAt = new Date(Date.now() + INVITE_TTL_MS);
+  const origin = process.env.APP_URL?.replace(/\/$/, "") || req.nextUrl.origin;
+  return `${origin}/invite/${token}`;
+}