Added a "Published in the last X days" filter, applied by the backend

GET /api/opportunities takes released_from, an RFC 3339 time, and lists
only the opportunities whose latest release is at or after it. It works
with paging and with changed_after, which keeps the same batch either
way; an invalid value is 400.

The frontend's filter defaults to 7 days, from midnight UK time, and
sends it with every load and catch-up, so a year of opportunities isn't
downloaded to show a week's. Changing the days loads the list again;
Clear filters goes back to 7.

internal/store/store_test.go

@@ -1020,7 +1020,9 @@ func TestChangeBatches(t *testing.T) {
 	}
 	changed := func(after string) ([]types.Opportunity, string) {
 		t.Helper()
-		opportunities, latest, err := s.ListChangedOpportunities(ctx, after)
+		opportunities, latest, err := s.ListChangedOpportunities(
+			ctx, after, time.Time{},
+		)
 		if err != nil {
 			t.Fatal(err)
 		}
@@ -1074,6 +1076,18 @@ func TestChangeBatches(t *testing.T) {
 		t.Errorf("changed opportunity without classifications: %+v", got[1])
 	}
 
+	// releasedFrom leaves out ocds-2 (released day 2), not the batch.
+	got, hash, err := s.ListChangedOpportunities(ctx, first, day(3))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !equal(uuids(got), []string{"ocds-3"}) || hash != second {
+		t.Errorf(
+			"changed from day 3: %v, %q; want [ocds-3], %q",
+			uuids(got), hash, second,
+		)
+	}
+
 	mustSetMarks(t, s, types.OpportunityMarks{
 		UUID:    "ocds-1",
 		Skipped: true,
@@ -1091,7 +1105,7 @@ func TestChangeBatches(t *testing.T) {
 		t.Errorf("changed after latest: %v, %q; want [], %q", got, again, hash)
 	}
 
-	_, _, err := s.ListChangedOpportunities(ctx, "unknown")
+	_, _, err = s.ListChangedOpportunities(ctx, "unknown", time.Time{})
 	if !errors.Is(err, store.ErrUnknownBatch) {
 		t.Errorf("unknown batch: err %v, want ErrUnknownBatch", err)
 	}