refactor: separate log timestamp from message as structured LogEntry

Introduced models.LogEntry{Message, CreatedAt} to replace raw strings
in the log pipeline. Timestamps are now formatted at render time, not
baked into stored messages.

- Engine: appendLog stores LogEntry with time.Now()
- Store: LoadLogs returns []LogEntry, selects created_at from DB
- Store: strips legacy [HH:MM] prefix from pre-refactor DB entries
- TUI: sidebar shows "MM/DD HH:MM" from CreatedAt
- TUI: full log view shows "MM/DD HH:MM" from CreatedAt
- SaveLog still receives plain message string (DB handles timestamp)

internal/monitor/monitor.go

@@ -40,7 +40,7 @@ type Engine struct {
 	liveState map[int]models.Site
 
 	logMu    sync.RWMutex
-	logStore []string
+	logStore []models.LogEntry
 
 	activeMu sync.RWMutex
 	isActive bool
@@ -152,11 +152,13 @@ func fmtDurationShort(d time.Duration) string {
 // appendLog adds a timestamped entry to the in-memory ring buffer and returns
 // it. It never touches the database, so it is safe to call from the db-write
 // drop/error path without recursing back through the write queue.
-func (e *Engine) appendLog(msg string) string {
-	ts := time.Now().Format("15:04:05")
-	entry := fmt.Sprintf("[%s] %s", ts, sanitizeLog(msg))
+func (e *Engine) appendLog(msg string) models.LogEntry {
+	entry := models.LogEntry{
+		Message:   sanitizeLog(msg),
+		CreatedAt: time.Now(),
+	}
 	e.logMu.Lock()
-	e.logStore = append([]string{entry}, e.logStore...)
+	e.logStore = append([]models.LogEntry{entry}, e.logStore...)
 	if len(e.logStore) > maxLogEntries {
 		e.logStore = e.logStore[:maxLogEntries]
 	}
@@ -166,7 +168,7 @@ func (e *Engine) appendLog(msg string) string {
 
 func (e *Engine) AddLog(msg string) {
 	entry := e.appendLog(msg)
-	e.enqueueWrite(writeLog{message: entry})
+	e.enqueueWrite(writeLog{message: entry.Message})
 }
 
 // enqueueWrite hands a persistence task to the writer goroutine without
@@ -246,16 +248,16 @@ func (e *Engine) Stop() {
 }
 
 func (e *Engine) InitLogs() {
-	logs, err := e.db.LoadLogs(context.Background(), maxLogEntries)
+	entries, err := e.db.LoadLogs(context.Background(), maxLogEntries)
 	if err != nil {
 		return
 	}
-	if len(logs) == 0 {
+	if len(entries) == 0 {
 		return
 	}
 	e.logMu.Lock()
 	defer e.logMu.Unlock()
-	e.logStore = logs
+	e.logStore = entries
 }
 
 // InitAlertHealth restores persisted alert send health so the dashboard shows real
@@ -278,10 +280,10 @@ func (e *Engine) InitAlertHealth() {
 	}
 }
 
-func (e *Engine) GetLogs() []string {
+func (e *Engine) GetLogs() []models.LogEntry {
 	e.logMu.RLock()
 	defer e.logMu.RUnlock()
-	logs := make([]string, len(e.logStore))
+	logs := make([]models.LogEntry, len(e.logStore))
 	copy(logs, e.logStore)
 	return logs
 }

internal/monitor/monitor_test.go

@@ -25,7 +25,7 @@ type mockStore struct {
 	sites         []models.SiteConfig
 	alerts        map[int]models.AlertConfig
 	maintenance   map[int]bool
-	logs          []string
+	logs          []models.LogEntry
 	history       map[int][]models.CheckRecord
 	savedChecks   []savedCheck
 	savedLogs     []string
@@ -103,7 +103,7 @@ func (m *mockStore) SaveLog(_ context.Context, msg string) error {
 	return nil
 }
 
-func (m *mockStore) LoadLogs(_ context.Context, _ int) ([]string, error) {
+func (m *mockStore) LoadLogs(_ context.Context, _ int) ([]models.LogEntry, error) {
 	return m.logs, nil
 }
 
@@ -330,7 +330,7 @@ func TestHandleStatusChange_AlertSuppressedMaintenance(t *testing.T) {
 	logs := e.GetLogs()
 	found := false
 	for _, l := range logs {
-		if containsStr(l, "suppressed") {
+		if containsStr(l.Message, "suppressed") {
 			found = true
 			break
 		}
@@ -973,14 +973,17 @@ func TestAddLog_PrependAndCap(t *testing.T) {
 	if len(logs) != 100 {
 		t.Errorf("expected 100 logs, got %d", len(logs))
 	}
-	if !containsStr(logs[0], "log-104") {
-		t.Errorf("expected newest log first, got %s", logs[0])
+	if !containsStr(logs[0].Message, "log-104") {
+		t.Errorf("expected newest log first, got %s", logs[0].Message)
 	}
 }
 
 func TestInitLogs_LoadsFromDB(t *testing.T) {
 	ms := newMockStore()
-	ms.logs = []string{"old-log-1", "old-log-2"}
+	ms.logs = []models.LogEntry{
+		{Message: "old-log-1"},
+		{Message: "old-log-2"},
+	}
 	e := newTestEngine(ms)
 	e.InitLogs()