feat(backend): implement category stack

Add category repo, service, handler, and wire all /categories endpoints
including list, create, get, update, delete, and list-tags.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/internal/service/category_service.go

@@ -0,0 +1,164 @@
+package service
+
+import (
+	"context"
+	"encoding/json"
+
+	"github.com/google/uuid"
+
+	"tanabata/backend/internal/domain"
+	"tanabata/backend/internal/port"
+)
+
+const categoryObjectType          = "category"
+const categoryObjectTypeID int16  = 3 // third row in 007_seed_data.sql object_types
+
+// CategoryParams holds the fields for creating or patching a category.
+type CategoryParams struct {
+	Name     string
+	Notes    *string
+	Color    *string         // nil = no change; pointer to empty string = clear
+	Metadata json.RawMessage
+	IsPublic *bool
+}
+
+// CategoryService handles category CRUD with ACL enforcement and audit logging.
+type CategoryService struct {
+	categories port.CategoryRepo
+	tags       port.TagRepo
+	acl        *ACLService
+	audit      *AuditService
+}
+
+// NewCategoryService creates a CategoryService.
+func NewCategoryService(
+	categories port.CategoryRepo,
+	tags port.TagRepo,
+	acl *ACLService,
+	audit *AuditService,
+) *CategoryService {
+	return &CategoryService{
+		categories: categories,
+		tags:       tags,
+		acl:        acl,
+		audit:      audit,
+	}
+}
+
+// ---------------------------------------------------------------------------
+// CRUD
+// ---------------------------------------------------------------------------
+
+// List returns a paginated, optionally filtered list of categories.
+func (s *CategoryService) List(ctx context.Context, params port.OffsetParams) (*domain.CategoryOffsetPage, error) {
+	return s.categories.List(ctx, params)
+}
+
+// Get returns a category by ID.
+func (s *CategoryService) Get(ctx context.Context, id uuid.UUID) (*domain.Category, error) {
+	return s.categories.GetByID(ctx, id)
+}
+
+// Create inserts a new category record.
+func (s *CategoryService) Create(ctx context.Context, p CategoryParams) (*domain.Category, error) {
+	userID, _, _ := domain.UserFromContext(ctx)
+
+	c := &domain.Category{
+		Name:      p.Name,
+		Notes:     p.Notes,
+		Color:     p.Color,
+		Metadata:  p.Metadata,
+		CreatorID: userID,
+	}
+	if p.IsPublic != nil {
+		c.IsPublic = *p.IsPublic
+	}
+
+	created, err := s.categories.Create(ctx, c)
+	if err != nil {
+		return nil, err
+	}
+
+	objType := categoryObjectType
+	_ = s.audit.Log(ctx, "category_create", &objType, &created.ID, nil)
+	return created, nil
+}
+
+// Update applies a partial patch to a category.
+func (s *CategoryService) Update(ctx context.Context, id uuid.UUID, p CategoryParams) (*domain.Category, error) {
+	userID, isAdmin, _ := domain.UserFromContext(ctx)
+
+	current, err := s.categories.GetByID(ctx, id)
+	if err != nil {
+		return nil, err
+	}
+
+	ok, err := s.acl.CanEdit(ctx, userID, isAdmin, current.CreatorID, categoryObjectTypeID, id)
+	if err != nil {
+		return nil, err
+	}
+	if !ok {
+		return nil, domain.ErrForbidden
+	}
+
+	patch := *current
+	if p.Name != "" {
+		patch.Name = p.Name
+	}
+	if p.Notes != nil {
+		patch.Notes = p.Notes
+	}
+	if p.Color != nil {
+		patch.Color = p.Color
+	}
+	if len(p.Metadata) > 0 {
+		patch.Metadata = p.Metadata
+	}
+	if p.IsPublic != nil {
+		patch.IsPublic = *p.IsPublic
+	}
+
+	updated, err := s.categories.Update(ctx, id, &patch)
+	if err != nil {
+		return nil, err
+	}
+
+	objType := categoryObjectType
+	_ = s.audit.Log(ctx, "category_edit", &objType, &id, nil)
+	return updated, nil
+}
+
+// Delete removes a category by ID, enforcing edit ACL.
+func (s *CategoryService) Delete(ctx context.Context, id uuid.UUID) error {
+	userID, isAdmin, _ := domain.UserFromContext(ctx)
+
+	c, err := s.categories.GetByID(ctx, id)
+	if err != nil {
+		return err
+	}
+
+	ok, err := s.acl.CanEdit(ctx, userID, isAdmin, c.CreatorID, categoryObjectTypeID, id)
+	if err != nil {
+		return err
+	}
+	if !ok {
+		return domain.ErrForbidden
+	}
+
+	if err := s.categories.Delete(ctx, id); err != nil {
+		return err
+	}
+
+	objType := categoryObjectType
+	_ = s.audit.Log(ctx, "category_delete", &objType, &id, nil)
+	return nil
+}
+
+// ---------------------------------------------------------------------------
+// Tags in category
+// ---------------------------------------------------------------------------
+
+// ListTags returns a paginated list of tags belonging to this category.
+func (s *CategoryService) ListTags(ctx context.Context, categoryID uuid.UUID, params port.OffsetParams) (*domain.TagOffsetPage, error) {
+	return s.tags.ListByCategory(ctx, categoryID, params)
+}