Made each source start from its newest stored release, with --since as an override

Without --since, fetcher service and job-search serve start each source
from the newest release stored for it (the new store.LatestReleaseDate,
by Source), so a restart carries on where it left off instead of fetching
the last day again; with nothing stored, the client's default still
applies. --since overrides it for every source. The Find a Tender source's
name is now a constant, as the Source its opportunities are stored with.

The README's guide to adding a source is brought up to date with the
current worker, start and config type names.

internal/fetcher/fetcher_test.go

@@ -552,3 +552,110 @@ func TestSave(t *testing.T) {
 		t.Errorf("save to a closed store = %v; want a saving error", err)
 	}
 }
+
+// updatedFromServer serves emptyPackage and sends the updatedFrom of each
+// request to the returned channel.
+func updatedFromServer(t *testing.T) (*httptest.Server, chan string) {
+	t.Helper()
+	updatedFrom := make(chan string, 10)
+	srv := httptest.NewServer(
+		http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			select {
+			case updatedFrom <- r.URL.Query().Get("updatedFrom"):
+			default:
+			}
+			io.WriteString(w, emptyPackage)
+		}),
+	)
+	t.Cleanup(srv.Close)
+	return srv, updatedFrom
+}
+
+// Without since, each source starts from its own newest stored release, or
+// with nothing stored from the client's default (the last day); since
+// overrides both.
+func TestRunStartsFromNewestStoredRelease(t *testing.T) {
+	opportunityStore := openTestStore(t)
+	srv, _ := ftsServer(t, http.StatusOK, onePackage)
+	stored, err := fetchers["FindATender"](
+		context.Background(), srv.URL, time.Now().Add(-time.Hour),
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+	_, err = opportunityStore.SaveOpportunities(context.Background(), stored)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	override := time.Date(2026, 9, 1, 0, 0, 0, 0, time.UTC)
+	tests := []struct {
+		name     string
+		since    time.Time
+		wantFAT  string
+		wantMock string
+	}{
+		// "" is the client's default: about a day ago.
+		{"stored", time.Time{}, "2026-10-01T12:00:00", ""},
+		{"override", override, "2026-09-01T00:00:00", "2026-09-01T00:00:00"},
+	}
+	for _, tt := range tests {
+		fat, fatFrom := updatedFromServer(t)
+		mock, mockFrom := updatedFromServer(t)
+		sources := &config.JSCSources{}
+		sources.FindATender.Enabled = true
+		sources.FindATender.Interval = time.Hour
+		sources.FindATender.APIRoot = fat.URL
+		sources.FTSMock.Enabled = true
+		sources.FTSMock.Interval = time.Hour
+		sources.FTSMock.APIRoot = mock.URL
+
+		ctx, cancel := context.WithCancel(context.Background())
+		done := make(chan error, 1)
+		go func() {
+			done <- New(sources, WithStore(opportunityStore)).Run(ctx, tt.since)
+		}()
+
+		for source, got := range map[string]chan string{
+			"FindATender": fatFrom,
+			"FTSMock":     mockFrom,
+		} {
+			want := tt.wantFAT
+			if source == "FTSMock" {
+				want = tt.wantMock
+			}
+			select {
+			case from := <-got:
+				if want == "" {
+					checkAboutADayAgo(t, from)
+				} else if from != want {
+					t.Errorf(
+						"%s: %s updatedFrom = %s; want %s",
+						tt.name, source, from, want,
+					)
+				}
+			case <-time.After(5 * time.Second):
+				t.Errorf("%s: %s was not polled", tt.name, source)
+			}
+		}
+		cancel()
+		<-done
+	}
+}
+
+// checkAboutADayAgo fails the test unless updatedFrom, in local time as the
+// client sends it, is about a day ago: within an hour, as a calendar day
+// across a clock change isn't 24 hours.
+func checkAboutADayAgo(t *testing.T, updatedFrom string) {
+	t.Helper()
+	from, err := time.ParseInLocation(
+		"2006-01-02T15:04:05", updatedFrom, time.Local,
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+	gap := time.Since(from) - 24*time.Hour
+	if gap < -time.Hour || gap > time.Hour {
+		t.Errorf("updatedFrom = %s; want about a day ago", updatedFrom)
+	}
+}