Add read endpoints for counter sales

The ingest only ever wrote. A bill that reached pos_orders was safe and
completely unreachable — no screen in the product could show it, and the
only way to see a day's counter takings was to query the database by
hand.

Three endpoints: a paged bill list, one bill with its lines, and a
summary split the three ways somebody actually asks for — by tender for
reconciling a drawer, by day for a chart, by till for an outlet running
several counters.

locationid is required on all of them and is the authorisation boundary,
so a caller cannot page through another shop's takings by omitting a
parameter. Fetching a bill under the wrong outlet returns 404 even when
the reference is a real one.

Dates match businessdate rather than arrival, because a till that was
offline overnight uploads yesterday's bills this morning and they belong
to yesterday. The list is ordered by billedat for the same reason —
sorting by arrival would interleave a recovered backlog through today.

Unlike the ingest handlers these answer in the usual envelope: they are
read by the web app, not by a terminal, and nothing about them is bound
to the till's contract.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

routes/posroutes.go

@@ -23,6 +23,12 @@ func RegisterPosRoutes(api fiber.Router, f *facade.Facade) {
 	pos.Post("/customers", f.PosController.IngestCustomers)
 	pos.Get("/catalogue", f.PosController.Catalogue)
 
+	// Counter sales, read back out. The ingest above only ever writes; without
+	// these a committed bill is unreachable from every screen in the product.
+	pos.Get("/sales", f.PosController.GetSales)
+	pos.Get("/sales/detail", f.PosController.GetSaleDetail)
+	pos.Get("/sales/summary", f.PosController.GetSalesSummary)
+
 	// Terminal presence, read from Redis. What the rider app's POS board and a
 	// support call both hit — the tills themselves publish health over the
 	// broker rather than posting it here.