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>

repositories/posRepository.go

@@ -37,6 +37,12 @@ type PosRepository interface {
 	IngestOrders(batch models.PosOrderBatch) (*models.PosAck, error)
 	IngestCustomers(batch models.PosCustomerBatch) (*models.PosAck, error)
 	Catalogue(storeID, since string, page, pageSize int) (*models.PosCatalogueResponse, error)
+
+	// Reading counter sales back out. Without these a committed bill is
+	// unreachable from every screen in the product.
+	Sales(f models.PosSalesFilter) (*models.PosSalesPage, error)
+	SaleDetail(locationID int, reference string) (*models.PosOrders, error)
+	SalesSummary(f models.PosSalesFilter) (*models.PosSalesSummary, error)
 }
 
 type posRepository struct {