store: return one calendar row per event

CalendarEvents range-scanned the key prefix and returned every historical
row, voided ones included. The facts table is append-only and the event
key is day plus summary, so moving a standup from 14:00 to 16:00 left two
rows under one key. The day plan prints a time per line, so it recited
both and told the owner he had two standups.

The query now drops voided rows, keeps the latest row within a source,
and prefers the best-evidenced source across them, so a notification
relay guessing at a meeting cannot displace the calendar read of it.
Found in review of #58.

internal/store/store_test.go

@@ -381,6 +381,66 @@ func TestCalendarEvents(t *testing.T) {
 	}
 }
 
+// A rescheduled meeting keeps its key and appends a row. The query must return
+// the current value, not the history: reciting both told the owner he had two
+// standups when one had been moved.
+func TestCalendarEventsReturnsOneRowPerEvent(t *testing.T) {
+	store := newTestStore(t)
+	defer store.Close()
+
+	ctx := context.Background()
+	day := time.Date(2026, 8, 3, 0, 0, 0, 0, time.UTC)
+	const key = "calendar_event_20260803_Standup"
+
+	store.WriteFact(ctx, day.Add(14*time.Hour), KindEnv, key,
+		`"Standup @ 14:00-14:30"`, calendar.SourcePersonal, 1.0, sql.NullInt64{})
+	store.WriteFact(ctx, day.Add(16*time.Hour), KindEnv, key,
+		`"Standup @ 16:00-16:30"`, calendar.SourcePersonal, 1.0, sql.NullInt64{})
+	// The notification relay guessed at the same meeting. A calendar read is
+	// better evidence, so the hedged row must not displace it.
+	store.WriteFact(ctx, day.Add(17*time.Hour), KindEnv, key,
+		`"Standup @ 17:00-17:30"`, calendar.SourceAmbient, calendar.AmbientConfidence, sql.NullInt64{})
+
+	events, err := store.CalendarEvents(ctx, day, day.AddDate(0, 0, 1))
+	if err != nil {
+		t.Fatalf("CalendarEvents: %v", err)
+	}
+	if len(events) != 1 {
+		t.Fatalf("got %d rows, want the current one only: %+v", len(events), events)
+	}
+	if events[0].Value != `"Standup @ 16:00-16:30"` {
+		t.Errorf("value = %q, want the latest calendar read", events[0].Value)
+	}
+}
+
+// A voided calendar fact is gone, not history to recite.
+func TestCalendarEventsSkipsVoidedRows(t *testing.T) {
+	store := newTestStore(t)
+	defer store.Close()
+
+	ctx := context.Background()
+	day := time.Date(2026, 8, 3, 0, 0, 0, 0, time.UTC)
+	id, err := store.WriteFact(ctx, day.Add(14*time.Hour), KindEnv, "calendar_event_20260803_Cancelled",
+		`"Cancelled @ 14:00-14:30"`, calendar.SourcePersonal, 1.0, sql.NullInt64{})
+	if err != nil {
+		t.Fatalf("WriteFact: %v", err)
+	}
+	if _, err := store.WriteFact(ctx, day.Add(15*time.Hour), KindEnv, "calendar_event_20260803_Cancelled",
+		`"Cancelled @ 14:00-14:30"`, calendar.SourcePersonal, 1.0, sql.NullInt64{Int64: id, Valid: true}); err != nil {
+		t.Fatalf("WriteFact void: %v", err)
+	}
+
+	events, err := store.CalendarEvents(ctx, day, day.AddDate(0, 0, 1))
+	if err != nil {
+		t.Fatalf("CalendarEvents: %v", err)
+	}
+	for _, e := range events {
+		if e.ID == id {
+			t.Fatalf("voided row %d came back: %+v", id, e)
+		}
+	}
+}
+
 // The work calendar arrives as relayed phone notifications, not a CalDAV read
 // (Vikunja #126). Those events belong in the same day's answer, and their
 // provenance has to survive the query so the caller can hedge them.