Conform Go code to project conventions

- Propagate context.Context through all exported store/service methods
  that perform I/O; use QueryContext/ExecContext/QueryRowContext throughout
- Add package-level sentinel errors (ErrNotFound, ErrAlreadyCheckedIn,
  ErrNotCheckedIn) and replace nil,nil returns with explicit errors
- Update handlers to use errors.Is() instead of nil checks, with correct
  HTTP status codes per error type
- Fix SQLite datetime('now') → MySQL NOW() in volunteer, schedule,
  timeoff, and checkin stores
- Refactor db.Migrate to execute schema statements individually (MySQL
  driver does not support multi-statement Exec)
- Fix import grouping in handler files (stdlib, external, internal)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/checkin/checkin.go

@@ -1,19 +1,26 @@
 package checkin
 
 import (
+	"context"
 	"database/sql"
 	"errors"
 	"fmt"
 	"time"
 )
 
+var (
+	ErrNotFound         = fmt.Errorf("check-in not found")
+	ErrAlreadyCheckedIn = fmt.Errorf("already checked in")
+	ErrNotCheckedIn     = fmt.Errorf("not checked in")
+)
+
 type CheckIn struct {
-	ID             int64      `json:"id"`
-	VolunteerID    int64      `json:"volunteer_id"`
-	ScheduleID     *int64     `json:"schedule_id,omitempty"`
-	CheckedInAt    time.Time  `json:"checked_in_at"`
-	CheckedOutAt   *time.Time `json:"checked_out_at,omitempty"`
-	Notes          string     `json:"notes,omitempty"`
+	ID           int64      `json:"id"`
+	VolunteerID  int64      `json:"volunteer_id"`
+	ScheduleID   *int64     `json:"schedule_id,omitempty"`
+	CheckedInAt  time.Time  `json:"checked_in_at"`
+	CheckedOutAt *time.Time `json:"checked_out_at,omitempty"`
+	Notes        string     `json:"notes,omitempty"`
 }
 
 type CheckInInput struct {
@@ -33,17 +40,17 @@ func NewStore(db *sql.DB) *Store {
 	return &Store{db: db}
 }
 
-func (s *Store) CheckIn(volunteerID int64, in CheckInInput) (*CheckIn, error) {
+func (s *Store) CheckIn(ctx context.Context, volunteerID int64, in CheckInInput) (*CheckIn, error) {
 	// Ensure no active check-in exists
 	var count int
-	s.db.QueryRow(
+	s.db.QueryRowContext(ctx,
 		`SELECT COUNT(*) FROM checkins WHERE volunteer_id = ? AND checked_out_at IS NULL`, volunteerID,
 	).Scan(&count)
 	if count > 0 {
-		return nil, fmt.Errorf("already checked in")
+		return nil, ErrAlreadyCheckedIn
 	}
 
-	res, err := s.db.Exec(
+	res, err := s.db.ExecContext(ctx,
 		`INSERT INTO checkins (volunteer_id, schedule_id, notes) VALUES (?, ?, ?)`,
 		volunteerID, in.ScheduleID, in.Notes,
 	)
@@ -51,43 +58,43 @@ func (s *Store) CheckIn(volunteerID int64, in CheckInInput) (*CheckIn, error) {
 		return nil, fmt.Errorf("insert checkin: %w", err)
 	}
 	id, _ := res.LastInsertId()
-	return s.GetByID(id)
+	return s.GetByID(ctx, id)
 }
 
-func (s *Store) CheckOut(volunteerID int64, in CheckOutInput) (*CheckIn, error) {
+func (s *Store) CheckOut(ctx context.Context, volunteerID int64, in CheckOutInput) (*CheckIn, error) {
 	var id int64
-	err := s.db.QueryRow(
+	err := s.db.QueryRowContext(ctx,
 		`SELECT id FROM checkins WHERE volunteer_id = ? AND checked_out_at IS NULL ORDER BY checked_in_at DESC LIMIT 1`,
 		volunteerID,
 	).Scan(&id)
 	if errors.Is(err, sql.ErrNoRows) {
-		return nil, fmt.Errorf("not checked in")
+		return nil, ErrNotCheckedIn
 	}
 	if err != nil {
 		return nil, fmt.Errorf("find active checkin: %w", err)
 	}
-	_, err = s.db.Exec(
-		`UPDATE checkins SET checked_out_at=datetime('now'), notes=COALESCE(NULLIF(?, ''), notes) WHERE id=?`,
+	_, err = s.db.ExecContext(ctx,
+		`UPDATE checkins SET checked_out_at=NOW(), notes=COALESCE(NULLIF(?, ''), notes) WHERE id=?`,
 		in.Notes, id,
 	)
 	if err != nil {
 		return nil, fmt.Errorf("checkout: %w", err)
 	}
-	return s.GetByID(id)
+	return s.GetByID(ctx, id)
 }
 
-func (s *Store) GetByID(id int64) (*CheckIn, error) {
+func (s *Store) GetByID(ctx context.Context, id int64) (*CheckIn, error) {
 	ci := &CheckIn{}
 	var checkedInAt string
 	var checkedOutAt sql.NullString
 	var scheduleID sql.NullInt64
 	var notes sql.NullString
 
-	err := s.db.QueryRow(
+	err := s.db.QueryRowContext(ctx,
 		`SELECT id, volunteer_id, schedule_id, checked_in_at, checked_out_at, notes FROM checkins WHERE id = ?`, id,
 	).Scan(&ci.ID, &ci.VolunteerID, &scheduleID, &checkedInAt, &checkedOutAt, &notes)
 	if errors.Is(err, sql.ErrNoRows) {
-		return nil, nil
+		return nil, ErrNotFound
 	}
 	if err != nil {
 		return nil, fmt.Errorf("get checkin: %w", err)
@@ -106,7 +113,7 @@ func (s *Store) GetByID(id int64) (*CheckIn, error) {
 	return ci, nil
 }
 
-func (s *Store) History(volunteerID int64) ([]CheckIn, error) {
+func (s *Store) History(ctx context.Context, volunteerID int64) ([]CheckIn, error) {
 	query := `SELECT id, volunteer_id, schedule_id, checked_in_at, checked_out_at, notes FROM checkins`
 	args := []any{}
 	if volunteerID > 0 {
@@ -115,7 +122,7 @@ func (s *Store) History(volunteerID int64) ([]CheckIn, error) {
 	}
 	query += ` ORDER BY checked_in_at DESC`
 
-	rows, err := s.db.Query(query, args...)
+	rows, err := s.db.QueryContext(ctx, query, args...)
 	if err != nil {
 		return nil, fmt.Errorf("list checkins: %w", err)
 	}