fix: take the package, image and chart version from the release tag

[Yash-Chindam]

What

The package, the API and the Helm chart all reported 0.1.0 while releases were at v0.17.0. Nothing could keep them in step, because releases are tagged by a workflow after merge and no file is edited to cut one.

How

The release tag is now the only version:

• The Python package takes its version from git tags (hatch-vcs); llm_router.__version__ and the API's reported version read it from the installed package.
• The image build has no git history, so CD passes the version in as a build argument. Built without it, the image reports 0.0.0 rather than guessing.
• The chart in the repository carries no version. The release workflow packages it with --version and --app-version set to the tag and attaches the .tgz to the GitHub release.
• .gitattributes fixes line endings to LF, so generated files compare equal on Windows and Linux.

Verified locally

• Installed package reports 0.17.1.dev0 (one commit past v0.17.0).
• docker build --build-arg VERSION=0.17.1 reports 0.17.1; without the argument, 0.0.0.
• helm package --version 0.17.1 --app-version 0.17.1 produces llm-routing-0.17.1.tgz.
• ruff and mypy clean, 499 tests pass, Playwright 9/9.

Limits

• The release workflow's new chart step runs for the first time when this merges; it is not exercised by CI.
• CD builds the image before the release is tagged, so its image is versioned as the previous release plus the distance from it (for example 0.17.0-1-gabc1234), not as the tag about to be cut.

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