feat: add artifacts and mime_types core modules

[Kamilbenkirane]

Fixes CEL-119

Adds two missing core modules to complete the core module migration:

• mime_types.py: MIME type enumerations (ImageMimeType, VideoMimeType, AudioMimeType)
• artifacts.py: Unified artifact types for media (Artifact, ImageArtifact, VideoArtifact, AudioArtifact)

These modules are prerequisites for migrating test files and enabling full core functionality.

Files Added:

• src/celeste/mime_types.py - MIME type enums following StrEnum pattern
• src/celeste/artifacts.py - Artifact classes with Pydantic BaseModel

Verification:

• Syntax validation passed
• Type checking passed (mypy)
• Linting passed
• Code style matches existing modules
• Docstrings follow CLAUDE.md guidelines (one-liners for simple classes)

Related:

• Part of CEL-119: Add supporting core modules
• Completes core module migration (all modules now in public repo)