refactor: eliminate store bypass and fix GetLastEvent style

- Add GetDistinctEventDates, UpdateEventWorkType, UpdateEventTime,
  DeleteEvent, GetEventByID store methods to calendar.go
- Replace all 5 raw h.DB.DB().Exec/Query calls with proper store methods
- Remove unused database/sql import from calendar.go
- Rewrite GetLastEvent to use QueryRow instead of Query+manual iteration
  (fixing row-vs-rows naming issue; removing rows.Close boilerplate)
- Add errors import to store.go for errors.Is

internal/bot/calendar.go

@@ -2,7 +2,6 @@ package bot
 
 import (
 	"context"
-	"database/sql"
 	"fmt"
 	"log/slog"
 	"strconv"
@@ -264,22 +263,11 @@ func (h *Handler) editCalendar(ctx context.Context, chatID int64, msgID int, yea
 
 	hasEvent := make(map[string]bool)
 	startDate, endDate := monthGregorianRange(cal, year, month)
-	rows, err := h.DB.DB().Query(
-		"SELECT DISTINCT date FROM days WHERE user_id=? AND date >= ? AND date <= ?",
-		user.ID, startDate, endDate,
-	)
+	dates, err := h.DB.GetDistinctEventDates(user.ID, startDate, endDate)
 	if err == nil {
-		for rows.Next() {
-			var d string
-			if err := rows.Scan(&d); err != nil {
-				slog.Error("failed to scan day row", "error", err)
-				continue
-			}
+		for _, d := range dates {
 			hasEvent[d] = true
 		}
-		if err := rows.Close(); err != nil {
-			slog.Error("failed to close rows", "error", err)
-		}
 	}
 
 	kbRows := [][]models.InlineKeyboardButton{}
@@ -439,8 +427,7 @@ func (h *Handler) editEventType(ctx context.Context, chatID int64, msgID int, us
 
 // setEventWorkType updates an event's work type and returns to the day view.
 func (h *Handler) setEventWorkType(ctx context.Context, chatID int64, msgID int, user *db.User, eventID, workTypeID int64, loc *time.Location) {
-	_, err := h.DB.DB().Exec("UPDATE events SET work_type_id=? WHERE id=?", workTypeID, eventID)
-	if err != nil {
+	if err := h.DB.UpdateEventWorkType(eventID, workTypeID); err != nil {
 		slog.Error("failed to update event work type", "event_id", eventID, "error", err)
 		h.sendDayView(ctx, chatID, msgID, user, "", loc, false)
 		return
@@ -557,8 +544,7 @@ func (h *Handler) editEventTimeSet(ctx context.Context, chatID int64, msgID int,
 		}
 	}
 
-	_, err = h.DB.DB().Exec("UPDATE events SET occurred_at=? WHERE id=?", newTime.Unix(), eventID)
-	if err != nil {
+	if err = h.DB.UpdateEventTime(eventID, newTime.Unix()); err != nil {
 		slog.Error("failed to update event occurred_at", "event_id", eventID, "error", err)
 		h.sendDayView(ctx, chatID, msgID, user, date, loc, false)
 		return
@@ -615,8 +601,7 @@ func (h *Handler) deleteEventConfirm(ctx context.Context, chatID int64, msgID in
 	t := time.Unix(event.OccurredAt, 0).In(loc)
 	date := t.Format(DateLayout)
 
-	_, err = h.DB.DB().Exec("DELETE FROM events WHERE id=?", eventID)
-	if err != nil {
+	if err = h.DB.DeleteEvent(eventID); err != nil {
 		slog.Error("failed to delete event", "event_id", eventID, "error", err)
 		h.sendDayView(ctx, chatID, msgID, user, date, loc, false)
 		return
@@ -654,19 +639,7 @@ func (h *Handler) backToDayView(ctx context.Context, chatID int64, msgID int, us
 
 // getEventByID fetches a single event by ID, scoped to the user.
 func (h *Handler) getEventByID(userID, eventID int64) (*db.Event, error) {
-	row := h.DB.DB().QueryRow(
-		"SELECT id, user_id, day_id, event_type, work_type_id, occurred_at, note, created_at FROM events WHERE id=? AND user_id=?",
-		eventID, userID,
-	)
-	var e db.Event
-	var wt sql.NullInt64
-	if err := row.Scan(&e.ID, &e.UserID, &e.DayID, &e.EventType, &wt, &e.OccurredAt, &e.Note, &e.CreatedAt); err != nil {
-		return nil, err
-	}
-	if wt.Valid {
-		e.WorkTypeID = &wt.Int64
-	}
-	return &e, nil
+	return h.DB.GetEventByID(eventID, userID)
 }
 
 // addEventTime shows an hour picker for adding a new event to a day.