Add a screen to set prices per branch

Branch price overrides were already applied at the till, in cash sales
and in M-Pesa, but nothing could set them.

- Products -> Branch prices (/admin/prices): pick a branch and set its
  price for any product; setting it back to the default removes the
  override, and "Use default" resets it. Branch managers see their own
  branch's prices read-only.
- /api/branch-prices: GET (owner: any branch; manager: own), PUT and
  DELETE (owner only). Branch and product must belong to the business;
  prices must be above zero; who changed a price is recorded.
- Branch pricing is a plan feature, as the Growth plan already
  advertised: Growth and Enterprise, plus any business on its free
  trial. A paid Starter business keeps any prices set during its trial
  and can reset them, but cannot set new ones.
- The catalogue view on the Products page always shows default prices;
  the till's view shows what the branch charges.

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

docs/PILOT_READINESS.md

@@ -76,7 +76,7 @@ the decisions taken along the way, and what is still open.
   sales from the old version are sent on the next sync.
 
 ## Testing
-- `npm test` runs 50 API tests against an in-memory MongoDB, covering role
+- `npm test` runs 56 API tests against an in-memory MongoDB, covering role
   enforcement, tenant isolation for sales, stock and transfers, suspension,
   the sync protocol (including a 120-sale backlog replayed twice), and the
   business day.
@@ -115,9 +115,23 @@ and checks every balance and total it predicts.
 2. ZERRO is trialling, so it can now add Branch B without a plan change.
 3. Create the manager and cashier logins with invite links from Staff.
 
+## Branch prices
+
+Owners set a different price per branch on **Products -> Branch prices**
+(`/admin/prices`, API `/api/branch-prices`). Setting a price back to the
+default removes the override. Branch managers see their own branch's
+prices read-only.
+
+- It is a Growth/Enterprise feature (as the plan list already advertised),
+  also available during the free trial. After a trial on Starter, existing
+  branch prices keep applying and can be reset, but not changed.
+- The till, cash sales and M-Pesa all read the branch price. A phone picks
+  up a change the next time it loads products; a sale it records offline in
+  the meantime is kept at the server's price and flagged as a price
+  mismatch.
+- The Products page always shows default prices.
+
 ## Still open
-- **Setting branch prices.** Branch price overrides are applied everywhere,
-  but there is no screen or API to set them yet.
 - **Global email and phone uniqueness.** One person cannot work for two
   businesses, and signup reveals whether an address is already registered.
 - **Isolation by convention.** Tenant filters are still written per route.