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/tui/update_test.go

@@ -300,7 +300,7 @@ func TestWriteDoneMsg_LogsErrorAndReloads(t *testing.T) {
 	mm := updated.(Model)
 	found := false
 	for _, line := range mm.engine.GetLogs() {
-		if strings.Contains(line, "Delete site failed: boom") {
+		if strings.Contains(line.Message, "Delete site failed: boom") {
 			found = true
 		}
 	}