feat: add batch upload, CLI, and exception hierarchy

- Add RbxError base class with AuthError, RateLimitError, UploadError,
  and AssetNotFoundError subclasses
- Add BatchUploadItem and BatchResult dataclasses
- Add batch_upload method with semaphore-limited concurrency (2 at a time)
- Add configurable max_attempts and poll_interval to upload_clothing_image
- Add CLI with upload and onsale commands (rbx-upload[cli] extra)
- Improve auth error handling across all client methods
- Bump version to 0.2.0

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "rbx-upload"
-version = "0.1.0"
+version = "0.2.0"
 description = "Roblox asset upload client"
 requires-python = ">=3.13"
 dependencies = [
@@ -8,6 +8,12 @@ dependencies = [
 ]
 license = "MIT"
 
+[project.optional-dependencies]
+cli = ["click>=8.0"]
+
+[project.scripts]
+rbx-upload = "rbx_upload.cli:main"
+
 [build-system]
 requires = ["hatchling"]
 build-backend = "hatchling.build"