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 and move them back if one
fails. The sweep prunes every file, one at a time under the target's
lock, and deletes a rotated file it leaves empty. Download lists the
files, then opens one at a time, oldest first, finding each again under
the target's current names, and gives each row its period. 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 {