Restructure into backend/ and frontend/ subprojects

- backend/ uses proper Python src layout (src/media_library_viewer_api/)
  with pyproject.toml, hatchling build, and PYTHONPATH=src convention
- frontend/ is a Vite + React + TypeScript SPA
- archive/ preserves the original Streamlit prototype for reference
- Cleaned up root to only contain docs, license, and subproject dirs
- Updated README for the new dual-subproject architecture

backend/pyproject.toml

@@ -16,9 +16,21 @@ dependencies = [
 [project.optional-dependencies]
 dev = ["httpx", "pytest", "pytest-asyncio", "ruff"]
 
+[build-system]
+requires = ["hatchling"]
+build-backend = "hatchling.build"
+
+[tool.hatch.build.targets.wheel]
+packages = ["src/media_library_viewer_api"]
+
 [tool.ruff]
 line-length = 120
 target-version = "py311"
+src = ["src"]
 
 [tool.ruff.lint]
 select = ["E", "F", "I", "W"]
+
+[tool.pytest.ini_options]
+testpaths = ["tests"]
+pythonpath = ["src"]