Rotate a database target's archive monthly, daily or hourly (closes #379)

A database target's rotation (none, monthly, daily or hourly) puts the
UTC period of each event's receive time in its archive file name, so
each file holds exactly its period's events. It is on the new webhook
page, the add target form and the target edit form, and shown in the
target list.

Renames move every one of a target's files, the sweep prunes every file
and deletes a rotated file it leaves empty, Download exports every file
oldest first with each row's period, and the target list names the
current file and totals the size of all of them.

Model: opus-5-5

internal/delivery/target_config_view_test.go

@@ -328,13 +328,49 @@ func TestNewTargetViews_Database(t *testing.T) {
 
 			assert.Equal(
 				t,
-				map[string]string{"Archive Expiry": tc.want},
+				map[string]string{
+					"Archive Expiry":   tc.want,
+					"Archive Rotation": rotationNone,
+				},
 				fieldMap(view.Config),
 			)
 		})
 	}
 }
 
+// TestNewTargetViews_DatabaseRotation proves the target list shows a
+// database target's rotation, none when it has none stored.
+func TestNewTargetViews_DatabaseRotation(t *testing.T) {
+	t.Parallel()
+
+	// Each stored config, and the rotation the list shows for it.
+	tests := map[string]string{
+		"":                rotationNone,
+		`{"rotation":""}`: rotationNone,
+	}
+
+	for _, rotation := range []string{
+		rotationNone, rotationMonthly, rotationDaily, rotationHourly,
+	} {
+		tests[`{"rotation":"`+rotation+`"}`] = rotation
+	}
+
+	for config, want := range tests {
+		t.Run(config, func(t *testing.T) {
+			t.Parallel()
+
+			view := viewFor(t, database.Target{
+				Type:   database.TargetTypeDatabase,
+				Config: config,
+			})
+
+			assert.Equal(
+				t, want, fieldMap(view.Config)["Archive Rotation"],
+			)
+		})
+	}
+}
+
 func TestNewTargetViews_Log(t *testing.T) {
 	t.Parallel()
 
@@ -382,6 +418,10 @@ func TestNewTargetViews_Unpresentable(t *testing.T) {
 			Type:   database.TargetTypeDatabase,
 			Config: `{"expiry":"a fortnight"}`,
 		},
+		"invalid archive rotation": {
+			Type:   database.TargetTypeDatabase,
+			Config: weeklyConfig,
+		},
 	}
 
 	for name, target := range tests {