fix: unify ErrNotFound — remove domain.ErrNotFound, use errors.ErrNotFound everywhere

Responds to PR #5 review comment: consolidate on common/errors.ErrNotFound
instead of having two separate error types (domain.PlainError and
common/errors.SimpleError) for the same sentinel.

- Remove domain.ErrNotFound and domain.PlainError type
- Keep domain.ErrNotImplemented as alias to common/errors.ErrNotImplemented
- Update sqlite_organization_repository to return cerrors.ErrNotFound
- Update sqlite_learning_category_repository to return cerrors.ErrNotFound
- Update server.go handleError to check only errors.ErrNotFound
- Update test to assert errors.ErrNotFound
- Remove unused domain import from server.go

internal/kurious/adapters/sqlite_learning_category_repository.go

@@ -8,6 +8,7 @@ import (
 	"log/slog"
 	"strings"
 
+	cerrors "git.loyso.art/frx/kurious/internal/common/errors"
 	"git.loyso.art/frx/kurious/internal/common/xslices"
 	"git.loyso.art/frx/kurious/internal/kurious/domain"
 
@@ -156,7 +157,7 @@ func (r *sqliteLearingCategoryRepository) Get(ctx context.Context, id string) (c
 	err = r.db.GetContext(ctx, &cdb, query, id)
 	if err != nil {
 		if errors.Is(err, sql.ErrNoRows) {
-			return domain.LearningCategory{}, domain.ErrNotFound
+			return domain.LearningCategory{}, cerrors.ErrNotFound
 		}
 		return domain.LearningCategory{}, fmt.Errorf("executing query: %w", err)
 	}

internal/kurious/adapters/sqlite_learning_category_repository_test.go

@@ -3,6 +3,7 @@ package adapters
 import (
 	"testing"
 
+	"git.loyso.art/frx/kurious/internal/common/errors"
 	"git.loyso.art/frx/kurious/internal/common/nullable"
 	"git.loyso.art/frx/kurious/internal/kurious/domain"
 
@@ -75,7 +76,7 @@ func (s *sqliteLearningCategoriesRepositorySuite) TestUpsert() {
 	const categoryID = "test-id-1"
 	repo := s.connection.LearningCategory()
 	gotCategory, err := repo.Get(s.ctx, categoryID)
-	s.ErrorIs(err, domain.ErrNotFound)
+	s.ErrorIs(err, errors.ErrNotFound)
 	s.Empty(gotCategory)
 
 	createdCategory := domain.LearningCategory{

internal/kurious/adapters/sqlite_organization_repository.go

@@ -9,6 +9,7 @@ import (
 	"strings"
 	"time"
 
+	cerrors "git.loyso.art/frx/kurious/internal/common/errors"
 	"git.loyso.art/frx/kurious/internal/common/xslices"
 	"git.loyso.art/frx/kurious/internal/kurious/domain"
 	"go.opentelemetry.io/otel/attribute"
@@ -228,7 +229,7 @@ func (r *sqliteOrganizationRepository) Get(ctx context.Context, params domain.Ge
 	err = r.db.GetContext(ctx, &orgdb, query, args...)
 	if err != nil {
 		if errors.Is(err, sql.ErrNoRows) {
-			return out, domain.ErrNotFound
+			return out, cerrors.ErrNotFound
 		}
 		return out, fmt.Errorf("executing query: %w", err)
 	}
@@ -306,14 +307,14 @@ func (r *sqliteOrganizationRepository) Delete(ctx context.Context, id string) (e
 	result, err := r.db.ExecContext(ctx, query, id)
 	if err != nil {
 		if errors.Is(err, sql.ErrNoRows) {
-			return domain.ErrNotFound
+			return cerrors.ErrNotFound
 		}
 		return fmt.Errorf("executing query: %w", err)
 	}
 
 	affected, _ := result.RowsAffected()
 	if affected == 0 {
-		return domain.ErrNotFound
+		return cerrors.ErrNotFound
 	}
 
 	return nil

internal/kurious/domain/error.go

@@ -1,12 +1,8 @@
 package domain
 
-const (
-	ErrNotFound       PlainError = "not found"
-	ErrNotImplemented PlainError = "not implemented"
+import (
+	cerrors "git.loyso.art/frx/kurious/internal/common/errors"
 )
 
-type PlainError string
-
-func (err PlainError) Error() string {
-	return string(err)
-}
+// ErrNotImplemented is delegated to common/errors.ErrNotImplemented for consistency.
+var ErrNotImplemented = cerrors.ErrNotImplemented

internal/kurious/ports/http/server.go

@@ -9,7 +9,6 @@ import (
 
 	"git.loyso.art/frx/kurious/internal/common/errors"
 	"git.loyso.art/frx/kurious/internal/common/xcontext"
-	"git.loyso.art/frx/kurious/internal/kurious/domain"
 	"git.loyso.art/frx/kurious/internal/kurious/service"
 	"git.loyso.art/frx/kurious/pkg/xdefault"
 
@@ -50,7 +49,7 @@ func handleError(ctx context.Context, err error, w http.ResponseWriter, log *slo
 	case stderrors.As(err, &valErr):
 		errorString = valErr.Error()
 		code = http.StatusBadRequest
-	case stderrors.Is(err, errors.ErrNotFound), stderrors.Is(err, domain.ErrNotFound):
+	case stderrors.Is(err, errors.ErrNotFound):
 		errorString = err.Error()
 		code = http.StatusNotFound
 	default: