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/schedule/schedule.go

@@ -1,12 +1,15 @@
 package schedule
 
 import (
+	"context"
 	"database/sql"
 	"errors"
 	"fmt"
 	"time"
 )
 
+var ErrNotFound = fmt.Errorf("schedule not found")
+
 type Schedule struct {
 	ID          int64     `json:"id"`
 	VolunteerID int64     `json:"volunteer_id"`
@@ -43,8 +46,8 @@ func NewStore(db *sql.DB) *Store {
 	return &Store{db: db}
 }
 
-func (s *Store) Create(in CreateInput) (*Schedule, error) {
-	res, err := s.db.Exec(
+func (s *Store) Create(ctx context.Context, in CreateInput) (*Schedule, error) {
+	res, err := s.db.ExecContext(ctx,
 		`INSERT INTO schedules (volunteer_id, title, starts_at, ends_at, notes) VALUES (?, ?, ?, ?, ?)`,
 		in.VolunteerID, in.Title, in.StartsAt, in.EndsAt, in.Notes,
 	)
@@ -52,18 +55,18 @@ func (s *Store) Create(in CreateInput) (*Schedule, error) {
 		return nil, fmt.Errorf("insert schedule: %w", err)
 	}
 	id, _ := res.LastInsertId()
-	return s.GetByID(id)
+	return s.GetByID(ctx, id)
 }
 
-func (s *Store) GetByID(id int64) (*Schedule, error) {
+func (s *Store) GetByID(ctx context.Context, id int64) (*Schedule, error) {
 	sc := &Schedule{}
 	var startsAt, endsAt, createdAt, updatedAt string
 	var notes sql.NullString
-	err := s.db.QueryRow(
+	err := s.db.QueryRowContext(ctx,
 		`SELECT id, volunteer_id, title, starts_at, ends_at, notes, created_at, updated_at FROM schedules WHERE id = ?`, id,
 	).Scan(&sc.ID, &sc.VolunteerID, &sc.Title, &startsAt, &endsAt, &notes, &createdAt, &updatedAt)
 	if errors.Is(err, sql.ErrNoRows) {
-		return nil, nil
+		return nil, ErrNotFound
 	}
 	if err != nil {
 		return nil, fmt.Errorf("get schedule: %w", err)
@@ -78,7 +81,7 @@ func (s *Store) GetByID(id int64) (*Schedule, error) {
 	return sc, nil
 }
 
-func (s *Store) List(volunteerID int64) ([]Schedule, error) {
+func (s *Store) List(ctx context.Context, volunteerID int64) ([]Schedule, error) {
 	query := `SELECT id, volunteer_id, title, starts_at, ends_at, notes, created_at, updated_at FROM schedules`
 	args := []any{}
 	if volunteerID > 0 {
@@ -87,7 +90,7 @@ func (s *Store) List(volunteerID int64) ([]Schedule, error) {
 	}
 	query += ` ORDER BY starts_at`
 
-	rows, err := s.db.Query(query, args...)
+	rows, err := s.db.QueryContext(ctx, query, args...)
 	if err != nil {
 		return nil, fmt.Errorf("list schedules: %w", err)
 	}
@@ -113,10 +116,10 @@ func (s *Store) List(volunteerID int64) ([]Schedule, error) {
 	return schedules, rows.Err()
 }
 
-func (s *Store) Update(id int64, in UpdateInput) (*Schedule, error) {
-	sc, err := s.GetByID(id)
-	if err != nil || sc == nil {
-		return sc, err
+func (s *Store) Update(ctx context.Context, id int64, in UpdateInput) (*Schedule, error) {
+	sc, err := s.GetByID(ctx, id)
+	if err != nil {
+		return nil, err
 	}
 	title := sc.Title
 	startsAt := sc.StartsAt.Format("2006-01-02 15:04:05")
@@ -135,17 +138,17 @@ func (s *Store) Update(id int64, in UpdateInput) (*Schedule, error) {
 	if in.Notes != nil {
 		notes = *in.Notes
 	}
-	_, err = s.db.Exec(
-		`UPDATE schedules SET title=?, starts_at=?, ends_at=?, notes=?, updated_at=datetime('now') WHERE id=?`,
+	_, err = s.db.ExecContext(ctx,
+		`UPDATE schedules SET title=?, starts_at=?, ends_at=?, notes=?, updated_at=NOW() WHERE id=?`,
 		title, startsAt, endsAt, notes, id,
 	)
 	if err != nil {
 		return nil, fmt.Errorf("update schedule: %w", err)
 	}
-	return s.GetByID(id)
+	return s.GetByID(ctx, id)
 }
 
-func (s *Store) Delete(id int64) error {
-	_, err := s.db.Exec(`DELETE FROM schedules WHERE id = ?`, id)
+func (s *Store) Delete(ctx context.Context, id int64) error {
+	_, err := s.db.ExecContext(ctx, `DELETE FROM schedules WHERE id = ?`, id)
 	return err
 }