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/errors.go

@@ -0,0 +1,13 @@
+package domain
+
+import "errors"
+
+// Sentinel domain errors. Handlers map these to HTTP status codes.
+var (
+	ErrNotFound        = errors.New("not found")
+	ErrForbidden       = errors.New("forbidden")
+	ErrUnauthorized    = errors.New("unauthorized")
+	ErrConflict        = errors.New("conflict")
+	ErrValidation      = errors.New("validation error")
+	ErrUnsupportedMIME = errors.New("unsupported MIME type")
+)