Added database.change_batch_retention, deleting old change batches

Each save that records a batch also deletes, in the same
transaction, the batches older than database.change_batch_retention
(30 days by default; 0 or less keeps them for ever), with their
records of opportunities. A client still holding a deleted batch
gets 410 Gone from ?changed_after= and reloads the whole list, as
for any unknown batch.

internal/config/conf_sections_test.go

@@ -595,18 +595,24 @@ func TestLoadFromViperInvalidValues(t *testing.T) {
 
 func TestDatabaseSection(t *testing.T) {
 	c := JSCDatabase{
-		File: "./jobsearch.db",
+		File:                 "./jobsearch.db",
+		ChangeBatchRetention: 48 * time.Hour,
 	}
 
-	if got, want := c.Keys(), []string{"file"}; !reflect.DeepEqual(got, want) {
-		t.Errorf("Keys = %v; want %v", got, want)
+	keys := []string{"file", "change_batch_retention"}
+	if got := c.Keys(); !reflect.DeepEqual(got, keys) {
+		t.Errorf("Keys = %v; want %v", got, keys)
 	}
 	if got, want := c.Map(), map[string]any{
-		"file": "./jobsearch.db",
+		"file":                   "./jobsearch.db",
+		"change_batch_retention": "48h0m0s",
 	}; !reflect.DeepEqual(got, want) {
 		t.Errorf("Map = %v; want %v", got, want)
 	}
-	want := map[string]any{"database.file": "./jobsearch.db"}
+	want := map[string]any{
+		"database.file":                   "./jobsearch.db",
+		"database.change_batch_retention": "48h0m0s",
+	}
 	if got := c.Flatten(); !reflect.DeepEqual(got, want) {
 		t.Errorf("Flatten = %v; want %v", got, want)
 	}
@@ -636,6 +642,19 @@ func TestDatabaseSection(t *testing.T) {
 		conf.Database.File != "./other.db" {
 		t.Errorf("SetValue(database.file) = %v, File = %q", err, conf.Database.File)
 	}
+
+	// 30 days by default.
+	retention := defaultConfig(t).Database.ChangeBatchRetention
+	if retention != 720*time.Hour {
+		t.Errorf("default change_batch_retention = %v; want 720h", retention)
+	}
+	err := conf.SetValue("database.change_batch_retention", "0s")
+	if err != nil || conf.Database.ChangeBatchRetention != 0 {
+		t.Errorf(
+			"SetValue(change_batch_retention) = %v, retention = %v",
+			err, conf.Database.ChangeBatchRetention,
+		)
+	}
 }
 
 func TestViperValue(t *testing.T) {