refactor: updated swagger;

feat: helper function in errors for checking postgres error types;
feat: sql query method for finding users by their email;
feat: registration begin/complete with checking existing username/email;
refactor: error handling in controller

backend/internal/database/query.sql.go

@@ -531,6 +531,25 @@ func (q *Queries) GetUserBansByUsername(ctx context.Context, username string) ([
 	return items, nil
 }
 
+const getUserByEmail = `-- name: GetUserByEmail :one
+SELECT users.id, users.username, users.verified, users.registration_date, users.deleted FROM users
+JOIN login_informations linfo ON linfo.user_id = users.id
+WHERE linfo.email = $1::text
+`
+
+func (q *Queries) GetUserByEmail(ctx context.Context, email string) (User, error) {
+	row := q.db.QueryRow(ctx, getUserByEmail, email)
+	var i User
+	err := row.Scan(
+		&i.ID,
+		&i.Username,
+		&i.Verified,
+		&i.RegistrationDate,
+		&i.Deleted,
+	)
+	return i, err
+}
+
 const getUserByLoginCredentials = `-- name: GetUserByLoginCredentials :one
 SELECT
   users.id,