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/models/User.ts

@@ -16,6 +16,11 @@ export interface IUser extends Document {
   isActive: boolean;
   failedLoginAttempts: number;
   lockedUntil: Date | null;
+  // Pending invite: the person sets their own password through a one-time
+  // link. Only the SHA-256 of the token is stored. Until they accept, the
+  // account has a random unknown password, so it cannot be signed into.
+  inviteTokenHash: string | null;
+  inviteExpiresAt: Date | null;
   createdAt: Date;
   updatedAt: Date;
 
@@ -34,6 +39,8 @@ const userSchema = new Schema<IUser>(
     isActive: { type: Boolean, default: true },
     failedLoginAttempts: { type: Number, default: 0 },
     lockedUntil: { type: Date, default: null },
+    inviteTokenHash: { type: String, default: null, index: { sparse: true } },
+    inviteExpiresAt: { type: Date, default: null },
   },
   { timestamps: true }
 );