refactor(store): split into per-aggregate files, unexport pool, remove dead ListBookIDs

backend/internal/store/users.go

@@ -0,0 +1,81 @@
+package store
+
+import (
+	"context"
+
+	"github.com/jackc/pgx/v5"
+)
+
+const userCols = "id, username, password_hash, role, created_at"
+
+func (s *Store) CountUsers(ctx context.Context) (int, error) {
+	var n int
+	err := s.p.QueryRow(ctx, "SELECT count(*) FROM users").Scan(&n)
+	return n, err
+}
+
+func (s *Store) CreateUser(ctx context.Context, username, hash, role string) (int64, error) {
+	var id int64
+	err := s.p.QueryRow(ctx,
+		"INSERT INTO users (username, password_hash, role) VALUES ($1,$2,$3) RETURNING id",
+		username, hash, role).Scan(&id)
+	return id, err
+}
+
+func (s *Store) GetUserByName(ctx context.Context, username string) (User, error) {
+	return scanUser(s.p.QueryRow(ctx, "SELECT "+userCols+" FROM users WHERE username=$1", username))
+}
+
+func (s *Store) GetUserByID(ctx context.Context, id int64) (User, error) {
+	return scanUser(s.p.QueryRow(ctx, "SELECT "+userCols+" FROM users WHERE id=$1", id))
+}
+
+func (s *Store) ListUsers(ctx context.Context) ([]User, error) {
+	rows, err := s.p.Query(ctx, "SELECT "+userCols+" FROM users ORDER BY id")
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	var out []User
+	for rows.Next() {
+		var u User
+		if err := rows.Scan(&u.ID, &u.Username, &u.PasswordHash, &u.Role, &u.CreatedAt); err != nil {
+			return nil, err
+		}
+		out = append(out, u)
+	}
+	return out, rows.Err()
+}
+
+// DeleteUser deletes a user with a transactional last-admin check (B5).
+// Returns ErrLastAdmin if the target is the last admin.
+func (s *Store) DeleteUser(ctx context.Context, id int64) error {
+	tx, err := s.p.Begin(ctx)
+	if err != nil {
+		return err
+	}
+	defer tx.Rollback(ctx)
+	var role string
+	if err := tx.QueryRow(ctx, "SELECT role FROM users WHERE id=$1 FOR UPDATE", id).Scan(&role); err != nil {
+		return err // includes pgx.ErrNoRows
+	}
+	if role == "admin" {
+		var n int
+		if err := tx.QueryRow(ctx, "SELECT count(*) FROM users WHERE role='admin'").Scan(&n); err != nil {
+			return err
+		}
+		if n <= 1 {
+			return ErrLastAdmin
+		}
+	}
+	if _, err := tx.Exec(ctx, "DELETE FROM users WHERE id=$1", id); err != nil {
+		return err
+	}
+	return tx.Commit(ctx)
+}
+
+func scanUser(row pgx.Row) (User, error) {
+	var u User
+	err := row.Scan(&u.ID, &u.Username, &u.PasswordHash, &u.Role, &u.CreatedAt)
+	return u, err
+}