Add Python 3.14 minimum version requirement

- Add .python-version file (for pyenv/uv) pinning to 3.14
- Add runtime version guard in scripts/optimize_image.py that exits with
  a clear error if run on Python < 3.14

Addresses review feedback on PR #106.

Author: mrbiggred

.python-version

@@ -0,0 +1 @@
+3.14

scripts/optimize_image.py

@@ -14,6 +14,14 @@
 
 import argparse
 import sys
+
+if sys.version_info < (3, 14):
+    print(
+        f"Error: Python 3.14 or later is required (running {sys.version}).",
+        file=sys.stderr,
+    )
+    sys.exit(1)
+
 from pathlib import Path
 
 try: