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>

messaging/posmqtt_test.go

@@ -56,6 +56,18 @@ func (f *fakePosService) TerminalHealth(context.Context, string) (map[string]str
 	return nil, nil
 }
 
+func (f *fakePosService) Sales(models.PosSalesFilter) (*models.PosSalesPage, error) {
+	return nil, nil
+}
+
+func (f *fakePosService) SaleDetail(int, string) (*models.PosOrders, error) {
+	return nil, nil
+}
+
+func (f *fakePosService) SalesSummary(models.PosSalesFilter) (*models.PosSalesSummary, error) {
+	return nil, nil
+}
+
 func (f *fakePosService) LocationHealth(context.Context, string) ([]map[string]string, error) {
 	return nil, nil
 }
@@ -100,9 +112,9 @@ func (c *fakeClient) Subscribe(string, byte, mqtt.MessageHandler) mqtt.Token {
 func (c *fakeClient) SubscribeMultiple(map[string]byte, mqtt.MessageHandler) mqtt.Token {
 	return doneToken{}
 }
-func (c *fakeClient) Unsubscribe(...string) mqtt.Token         { return doneToken{} }
-func (c *fakeClient) AddRoute(string, mqtt.MessageHandler)     {}
-func (c *fakeClient) OptionsReader() mqtt.ClientOptionsReader  { return mqtt.ClientOptionsReader{} }
+func (c *fakeClient) Unsubscribe(...string) mqtt.Token        { return doneToken{} }
+func (c *fakeClient) AddRoute(string, mqtt.MessageHandler)    {}
+func (c *fakeClient) OptionsReader() mqtt.ClientOptionsReader { return mqtt.ClientOptionsReader{} }
 
 type doneToken struct{}