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/auth/auth.go

@@ -1,6 +1,7 @@
 package auth
 
 import (
+	"context"
 	"database/sql"
 	"errors"
 	"fmt"
@@ -27,10 +28,10 @@ func NewService(db *sql.DB, secret string) *Service {
 	return &Service{db: db, jwtSecret: []byte(secret)}
 }
 
-func (s *Service) Login(email, password string) (string, error) {
+func (s *Service) Login(ctx context.Context, email, password string) (string, error) {
 	var id int64
 	var hash, role string
-	err := s.db.QueryRow(
+	err := s.db.QueryRowContext(ctx,
 		`SELECT id, password, role FROM volunteers WHERE email = ? AND active = 1`,
 		email,
 	).Scan(&id, &hash, &role)