build: derive package version from git via setuptools-scm

Replace hatchling and the static version with setuptools + setuptools-scm,
following Kaya's pre-uv packaging setup: dynamic version, generated
_version.py, and release/* tag prefix. The Docker builder now includes git
and the .git directory so the image is versioned from the checkout. CI
checks out full history and tags for the tag-triggered builds.

Dockerfile

@@ -6,6 +6,9 @@
 FROM python:3.14-alpine3.24 AS builder
 COPY --from=ghcr.io/astral-sh/uv:0.13.1 /uv /uvx /bin/
 
+# setuptools-scm derives the package version from git history and tags.
+RUN apk add --no-cache git
+
 ENV UV_COMPILE_BYTECODE=1 \
     UV_LINK_MODE=copy \
     UV_PYTHON_DOWNLOADS=never
@@ -17,9 +20,11 @@ COPY pyproject.toml uv.lock .python-version ./
 RUN --mount=type=cache,target=/root/.cache/uv \
     uv sync --frozen --no-dev --no-install-project
 
-# Then install the project itself as a regular wheel.
+# Then install the project itself as a regular wheel. The git metadata is
+# required by setuptools-scm to derive the package version.
 COPY README.md ./
 COPY src ./src
+COPY .git ./.git
 RUN --mount=type=cache,target=/root/.cache/uv \
     uv sync --frozen --no-dev --no-editable