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/timeoff/timeoff.go

@@ -1,12 +1,15 @@
 package timeoff
 
 import (
+	"context"
 	"database/sql"
 	"errors"
 	"fmt"
 	"time"
 )
 
+var ErrNotFound = fmt.Errorf("time off request not found")
+
 type Request struct {
 	ID          int64      `json:"id"`
 	VolunteerID int64      `json:"volunteer_id"`
@@ -38,8 +41,8 @@ func NewStore(db *sql.DB) *Store {
 	return &Store{db: db}
 }
 
-func (s *Store) Create(volunteerID int64, in CreateInput) (*Request, error) {
-	res, err := s.db.Exec(
+func (s *Store) Create(ctx context.Context, volunteerID int64, in CreateInput) (*Request, error) {
+	res, err := s.db.ExecContext(ctx,
 		`INSERT INTO time_off_requests (volunteer_id, starts_at, ends_at, reason) VALUES (?, ?, ?, ?)`,
 		volunteerID, in.StartsAt, in.EndsAt, in.Reason,
 	)
@@ -47,22 +50,22 @@ func (s *Store) Create(volunteerID int64, in CreateInput) (*Request, error) {
 		return nil, fmt.Errorf("insert time off request: %w", err)
 	}
 	id, _ := res.LastInsertId()
-	return s.GetByID(id)
+	return s.GetByID(ctx, id)
 }
 
-func (s *Store) GetByID(id int64) (*Request, error) {
+func (s *Store) GetByID(ctx context.Context, id int64) (*Request, error) {
 	req := &Request{}
 	var startsAt, endsAt, createdAt, updatedAt string
 	var reason sql.NullString
 	var reviewedBy sql.NullInt64
 	var reviewedAt sql.NullString
 
-	err := s.db.QueryRow(
+	err := s.db.QueryRowContext(ctx,
 		`SELECT id, volunteer_id, starts_at, ends_at, reason, status, reviewed_by, reviewed_at, created_at, updated_at
 		 FROM time_off_requests WHERE id = ?`, id,
 	).Scan(&req.ID, &req.VolunteerID, &startsAt, &endsAt, &reason, &req.Status, &reviewedBy, &reviewedAt, &createdAt, &updatedAt)
 	if errors.Is(err, sql.ErrNoRows) {
-		return nil, nil
+		return nil, ErrNotFound
 	}
 	if err != nil {
 		return nil, fmt.Errorf("get time off request: %w", err)
@@ -84,7 +87,7 @@ func (s *Store) GetByID(id int64) (*Request, error) {
 	return req, nil
 }
 
-func (s *Store) List(volunteerID int64) ([]Request, error) {
+func (s *Store) List(ctx context.Context, volunteerID int64) ([]Request, error) {
 	query := `SELECT id, volunteer_id, starts_at, ends_at, reason, status, reviewed_by, reviewed_at, created_at, updated_at FROM time_off_requests`
 	args := []any{}
 	if volunteerID > 0 {
@@ -93,7 +96,7 @@ func (s *Store) List(volunteerID int64) ([]Request, error) {
 	}
 	query += ` ORDER BY starts_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 time off requests: %w", err)
 	}
@@ -128,13 +131,13 @@ func (s *Store) List(volunteerID int64) ([]Request, error) {
 	return requests, rows.Err()
 }
 
-func (s *Store) Review(id, reviewerID int64, status string) (*Request, error) {
-	_, err := s.db.Exec(
-		`UPDATE time_off_requests SET status=?, reviewed_by=?, reviewed_at=datetime('now'), updated_at=datetime('now') WHERE id=?`,
+func (s *Store) Review(ctx context.Context, id, reviewerID int64, status string) (*Request, error) {
+	_, err := s.db.ExecContext(ctx,
+		`UPDATE time_off_requests SET status=?, reviewed_by=?, reviewed_at=NOW(), updated_at=NOW() WHERE id=?`,
 		status, reviewerID, id,
 	)
 	if err != nil {
 		return nil, fmt.Errorf("review time off request: %w", err)
 	}
-	return s.GetByID(id)
+	return s.GetByID(ctx, id)
 }