Derive the version from git with verri

Use verri's strawberry flavour through hatchling's code version source, so
versions look like 2026.10.21.0: the HEAD commit date plus the number of
earlier commits that day.

Container images build the sdist in a separate stage that has the git
repository, so they get the same version instead of a +vcs.missing one.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

version.py

@@ -0,0 +1,5 @@
+# SPDX-FileCopyrightText: 2026 Bart Broere
+# SPDX-License-Identifier: GPL-3.0-or-later
+"""Build-time version source for hatchling, see [tool.hatch.version]."""
+
+from verri.tasty import strawberry  # noqa: F401