WIP: root background loops at context.Background() (refs #97)

Incomplete: tests pass but three lint findings remain (funcorder on
registerHooks, unparam in engine_integration_test.go). Committed to
preserve work in progress; to be amended into a single clean commit.

internal/delivery/export_test.go

@@ -7,6 +7,7 @@ import (
 	"net/http"
 	"time"
 
+	"go.uber.org/fx"
 	"gorm.io/gorm"
 	"sneak.berlin/go/webhooker/internal/database"
 )
@@ -189,8 +190,16 @@ func (e *Engine) ExportRecoverInFlight(
 }
 
 // ExportStart exposes start for testing.
-func (e *Engine) ExportStart(ctx context.Context) {
-	e.start(ctx)
+func (e *Engine) ExportStart() {
+	e.start()
+}
+
+// ExportRegisterHooks registers the engine's real fx lifecycle
+// hooks on a lifecycle supplied by a test, so a test can drive
+// the exact OnStart/OnStop functions the application runs and
+// hand OnStart the kind of context fx actually supplies.
+func (e *Engine) ExportRegisterHooks(lc fx.Lifecycle) {
+	e.registerHooks(lc)
 }
 
 // ExportStop exposes stop for testing.