Add JobStatus enum for API response status values

[laurencegoolsby]

Context

API responses use raw strings for job_status ("completed", "processing", "failed", "not_supported"). These are mapped from internal ProcessStatus values in build_v1_api_response but have no type enforcement.

Relates to #37

Implementation

• Create JobStatus(StrEnum) with values: COMPLETED, PROCESSING, FAILED, NOT_SUPPORTED
• Use as the type for JobStatusResponse.job_status and UploadAsyncResponse.job_status
• Update build_v1_api_response to return JobStatus enum values instead of raw strings