Migrate to pyproject.toml with setuptools_scm

Replace custom version management with setuptools_scm to eliminate
source file modifications during installation. Version is now derived
from git tags at build time without writing to tracked files.

Changes:
- Add pyproject.toml with full project metadata
- Simplify setup.py to minimal backwards-compatible shim
- Remove curateipsum/_version.py from version control
- Add _version.py to .gitignore (auto-generated at build time)
- Use PEP 440 compliant version strings

Fixes #18

pyproject.toml

@@ -0,0 +1,36 @@
+[build-system]
+requires = ["setuptools>=64", "setuptools_scm>=8"]
+build-backend = "setuptools.build_meta"
+
+[project]
+name = "cura-te-ipsum"
+dynamic = ["version"]
+description = "Backup utility"
+authors = [
+    {name = "Maks Snegov", email = "snegov@spqr.link"}
+]
+readme = "README.md"
+requires-python = ">=3.6"
+classifiers = [
+    "Development Status :: 2 - Pre-Alpha",
+    "Intended Audience :: Developers",
+    "Intended Audience :: System Administrators",
+    "License :: OSI Approved :: GNU General Public License v3 (GPLv3)",
+    "Operating System :: OS Independent",
+    "Programming Language :: Python :: 3",
+    "Topic :: System :: Archiving :: Backup",
+]
+
+[project.urls]
+Homepage = "https://github.com/snegov/cura-te-ipsum"
+"Bug Tracker" = "https://github.com/snegov/cura-te-ipsum/issues"
+GitHub = "https://github.com/snegov/cura-te-ipsum"
+
+[project.scripts]
+cura-te-ipsum = "curateipsum.cli:main"
+
+[tool.setuptools]
+packages = ["curateipsum"]
+
+[tool.setuptools_scm]
+version_file = "curateipsum/_version.py"