feat(backend): initialize Go module and implement domain layer

- Add go.mod (module tanabata/backend, Go 1.21) with uuid dependency
- Implement internal/domain: File, Tag, TagRule, Category, Pool, PoolFile,
  User, Session, Permission, ObjectType, AuditEntry + all pagination types
- Add domain error sentinels (ErrNotFound, ErrForbidden, etc.)
- Add context helpers WithUser/UserFromContext for JWT propagation
- Fix migration: remove redundant DEFAULT on exif jsonb column

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

backend/internal/domain/category.go

@@ -0,0 +1,21 @@
+package domain
+
+import (
+	"encoding/json"
+	"time"
+
+	"github.com/google/uuid"
+)
+
+// Category is a logical grouping of tags.
+type Category struct {
+	ID          uuid.UUID
+	Name        string
+	Notes       *string
+	Color       *string // 6-char hex
+	Metadata    json.RawMessage
+	CreatorID   int16
+	CreatorName string // denormalized
+	IsPublic    bool
+	CreatedAt   time.Time // extracted from UUID v7
+}