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>

models/posorder.go

@@ -136,3 +136,74 @@ type PosOrderItems struct {
 func (PosOrderItems) TableName() string {
 	return "pos_order_items"
 }
+
+// PosSalesFilter scopes a query over counter sales.
+//
+// Locationid is required and is the authorisation boundary — every read is
+// scoped to one outlet, so a caller cannot page through another shop's takings
+// by omitting a parameter.
+type PosSalesFilter struct {
+	Locationid  int
+	Fromdate    string // YYYY-MM-DD, matched against businessdate
+	Todate      string
+	Terminalid  string
+	Cashiername string
+	Paymentmode string
+	Pageno      int
+	Pagesize    int
+}
+
+// PosSalesPage is one page of bills, with the total so a caller can paginate
+// without a second request.
+type PosSalesPage struct {
+	Total    int64       `json:"total"`
+	Pageno   int         `json:"pageno"`
+	Pagesize int         `json:"pagesize"`
+	Bills    []PosOrders `json:"bills"`
+}
+
+// PosSalesSummary totals a range of counter sales.
+//
+// Deliberately separate from the bill list: a shop settling a till wants the
+// figures, not five hundred rows, and computing them client-side would mean
+// fetching every page first.
+type PosSalesSummary struct {
+	Locationid   int     `json:"locationid"`
+	Fromdate     string  `json:"fromdate"`
+	Todate       string  `json:"todate"`
+	Billcount    int     `json:"billcount"`
+	Itemcount    int     `json:"itemcount"`
+	Grosssales   float64 `json:"grosssales"`
+	Taxcollected float64 `json:"taxcollected"`
+	Discount     float64 `json:"discountgiven"`
+	Roundoff     float64 `json:"roundoff"`
+	Averagebill  float64 `json:"averagebill"`
+
+	// What a cashier reconciles the drawer against.
+	Bypaymentmode []PosPaymentTotal `json:"bypaymentmode"`
+
+	// One row per trading day, for a chart.
+	Byday []PosDayTotal `json:"byday"`
+
+	// Which tills contributed, so an outlet with several counters can see them
+	// apart without a second query.
+	Byterminal []PosTerminalTotal `json:"byterminal"`
+}
+
+type PosPaymentTotal struct {
+	Paymentmode string  `json:"paymentmode"`
+	Billcount   int     `json:"billcount"`
+	Amount      float64 `json:"amount"`
+}
+
+type PosDayTotal struct {
+	Businessdate string  `json:"businessdate"`
+	Billcount    int     `json:"billcount"`
+	Amount       float64 `json:"amount"`
+}
+
+type PosTerminalTotal struct {
+	Terminalid string  `json:"terminalid"`
+	Billcount  int     `json:"billcount"`
+	Amount     float64 `json:"amount"`
+}