fix: bump __version__ to 0.3.0 to match the released version

[Niko11111]

__version__.py still reports 0.2.0, although v0.3.0 was released on
23.12.2025. This is the file reported in #89.

It is not only cosmetic. .github/workflows/helm-publish.yml derives the
chart's appVersion from this file on every push to main:

BASE_VERSION=$(python -c "exec(open('__version__.py').read()); print(__version__)")
APP_VERSION="${BASE_VERSION}-dev"

So every dev chart published since the v0.3.0 release carries
appVersion: 0.2.0-dev. Users who check the version the way #89 describes
(docker exec openspoolman cat __version__.py) are told the wrong thing too.

Release builds are unaffected: on a release the workflow takes the version
from the git tag, not from this file.

This PR only corrects the value. Showing the version in the web UI, which
#89 also asks for, is a separate change.

PR checklist

• Scope is minimal; no unrelated refactors - one line, line endings preserved
• No secrets or sensitive values introduced
• Errors handled (timeouts, retries/backoff if applicable) - n/a
• Tests added/updated (or justification if not) - no behaviour to test; verified
  that python -c "exec(open('__version__.py').read()); print(__version__)",
  the exact call helm-publish.yml makes, now prints 0.3.0
• README/config updated if behavior or configuration changed - n/a, the README
  News section already lists v0.3.0
• Docker/Helm impact considered (ports, env vars, volumes) - this is the Helm
  impact; dev charts will now report 0.3.0-dev
• Filament matching rules preserved (or explicitly enhanced with tests) - untouched

Verified locally: pytest green in both matrix legs (TRACK_LAYER_USAGE
false and true) on Python 3.13, and flake8 --select=E9,F63,F7,F82 clean.