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/volunteer/volunteer.go

@@ -1,12 +1,15 @@
 package volunteer
 
 import (
+	"context"
 	"database/sql"
 	"errors"
 	"fmt"
 	"time"
 )
 
+var ErrNotFound = fmt.Errorf("volunteer not found")
+
 type Volunteer struct {
 	ID        int64     `json:"id"`
 	Name      string    `json:"name"`
@@ -39,8 +42,8 @@ func NewStore(db *sql.DB) *Store {
 	return &Store{db: db}
 }
 
-func (s *Store) Create(name, email, hashedPassword, role string) (*Volunteer, error) {
-	res, err := s.db.Exec(
+func (s *Store) Create(ctx context.Context, name, email, hashedPassword, role string) (*Volunteer, error) {
+	res, err := s.db.ExecContext(ctx,
 		`INSERT INTO volunteers (name, email, password, role) VALUES (?, ?, ?, ?)`,
 		name, email, hashedPassword, role,
 	)
@@ -48,17 +51,17 @@ func (s *Store) Create(name, email, hashedPassword, role string) (*Volunteer, er
 		return nil, fmt.Errorf("insert volunteer: %w", err)
 	}
 	id, _ := res.LastInsertId()
-	return s.GetByID(id)
+	return s.GetByID(ctx, id)
 }
 
-func (s *Store) GetByID(id int64) (*Volunteer, error) {
+func (s *Store) GetByID(ctx context.Context, id int64) (*Volunteer, error) {
 	v := &Volunteer{}
 	var createdAt, updatedAt string
-	err := s.db.QueryRow(
+	err := s.db.QueryRowContext(ctx,
 		`SELECT id, name, email, role, active, created_at, updated_at FROM volunteers WHERE id = ?`, id,
 	).Scan(&v.ID, &v.Name, &v.Email, &v.Role, &v.Active, &createdAt, &updatedAt)
 	if errors.Is(err, sql.ErrNoRows) {
-		return nil, nil
+		return nil, ErrNotFound
 	}
 	if err != nil {
 		return nil, fmt.Errorf("get volunteer: %w", err)
@@ -68,14 +71,14 @@ func (s *Store) GetByID(id int64) (*Volunteer, error) {
 	return v, nil
 }
 
-func (s *Store) List(activeOnly bool) ([]Volunteer, error) {
+func (s *Store) List(ctx context.Context, activeOnly bool) ([]Volunteer, error) {
 	query := `SELECT id, name, email, role, active, created_at, updated_at FROM volunteers`
 	if activeOnly {
 		query += ` WHERE active = 1`
 	}
 	query += ` ORDER BY name`
 
-	rows, err := s.db.Query(query)
+	rows, err := s.db.QueryContext(ctx, query)
 	if err != nil {
 		return nil, fmt.Errorf("list volunteers: %w", err)
 	}
@@ -95,10 +98,10 @@ func (s *Store) List(activeOnly bool) ([]Volunteer, error) {
 	return volunteers, rows.Err()
 }
 
-func (s *Store) Update(id int64, in UpdateInput) (*Volunteer, error) {
-	v, err := s.GetByID(id)
-	if err != nil || v == nil {
-		return v, err
+func (s *Store) Update(ctx context.Context, id int64, in UpdateInput) (*Volunteer, error) {
+	v, err := s.GetByID(ctx, id)
+	if err != nil {
+		return nil, err
 	}
 	if in.Name != nil {
 		v.Name = *in.Name
@@ -116,12 +119,12 @@ func (s *Store) Update(id int64, in UpdateInput) (*Volunteer, error) {
 	if v.Active {
 		activeInt = 1
 	}
-	_, err = s.db.Exec(
-		`UPDATE volunteers SET name=?, email=?, role=?, active=?, updated_at=datetime('now') WHERE id=?`,
+	_, err = s.db.ExecContext(ctx,
+		`UPDATE volunteers SET name=?, email=?, role=?, active=?, updated_at=NOW() WHERE id=?`,
 		v.Name, v.Email, v.Role, activeInt, id,
 	)
 	if err != nil {
 		return nil, fmt.Errorf("update volunteer: %w", err)
 	}
-	return s.GetByID(id)
+	return s.GetByID(ctx, id)
 }