Refactoring complete: v0.4.0 architectural milestone (#1)

## Summary

Completed Phase 5 refactoring and reached the target architecture.

**Architectural milestone achieved:**
- Service layer owns all state and is the sole writer
- UI is a thin Fyne view, all widget updates marshaled via `fyne.Do`
- Core engines are stateless and injectable
- Domain types are pure (no `yaml:"-"` fields)
- Full module builds and `go vet ./...` clean

## Changes

- Bump version: 0.3.6 → 0.4.0
- Update CHANGELOG with Phase 5 summary
- Add ROADMAP "Refactoring Follow-Ups" section

## Known follow-up work

1. **Linux test build broken** — `runner_test.go` needs `//go:build windows` tag
2. **File-size limits exceeded** — `operations.go` (486 lines), `jobs_view.go` (415 lines)

See ROADMAP.md for details.

---------

Co-authored-by: mixeme <mix.public@ya.ru>
Reviewed-on: #1

src/app/events_test.go

@@ -0,0 +1,59 @@
+package app
+
+import (
+	"testing"
+
+	"gitea.mixdep.ru/mix/gosentry/src/domain"
+)
+
+func TestEmitDeliversToAllObserversInOrder(t *testing.T) {
+	svc := newTestService(nil)
+
+	var first, second []Event
+	svc.Subscribe(ObserverFunc(func(e Event) { first = append(first, e) }))
+	svc.Subscribe(ObserverFunc(func(e Event) { second = append(second, e) }))
+
+	svc.emit(JobChanged{JobID: 7})
+	svc.emit(RunRecorded{Record: domain.RunRecord{JobID: 7, State: "Success"}})
+	svc.emit(SchedulerStateChanged{Paused: true})
+
+	for name, got := range map[string][]Event{"first": first, "second": second} {
+		if len(got) != 3 {
+			t.Fatalf("%s observer got %d events, want 3", name, len(got))
+		}
+		if jc, ok := got[0].(JobChanged); !ok || jc.JobID != 7 {
+			t.Errorf("%s event[0] = %#v, want JobChanged{JobID:7}", name, got[0])
+		}
+		if rr, ok := got[1].(RunRecorded); !ok || rr.Record.State != "Success" {
+			t.Errorf("%s event[1] = %#v, want RunRecorded Success", name, got[1])
+		}
+		if ss, ok := got[2].(SchedulerStateChanged); !ok || !ss.Paused {
+			t.Errorf("%s event[2] = %#v, want SchedulerStateChanged{Paused:true}", name, got[2])
+		}
+	}
+}
+
+func TestEmitWithNoObserversIsNoop(t *testing.T) {
+	svc := newTestService(nil)
+	// Must not panic with an empty observer list.
+	svc.emit(JobChanged{})
+}
+
+// Observers may read Service state from within OnEvent without deadlocking,
+// because emit is called outside the state lock.
+func TestObserverCanReadServiceState(t *testing.T) {
+	jobs := []domain.Job{{ID: 1, Name: "Job", Enabled: true}}
+	svc := newTestService(jobs)
+
+	var sawName string
+	svc.Subscribe(ObserverFunc(func(Event) {
+		if snapshot := svc.Jobs(); len(snapshot) == 1 {
+			sawName = snapshot[0].Name
+		}
+	}))
+
+	svc.emit(JobChanged{JobID: 1})
+	if sawName != "Job" {
+		t.Errorf("observer read name = %q, want %q", sawName, "Job")
+	}
+}