Configure bumpver for Concourse release pipeline

[blarghmatey]

What are the relevant tickets?

N/A

Description (What does it do?)

Adds [tool.bumpver] configuration to pyproject.toml so the Concourse
release pipeline can update the application version automatically on each
release.

The new version format is YYYY.MM.DD.N (e.g., 2026.04.16.1), which
replaces the previous semver-style version strings. This is required by the
Concourse release resource workflow being rolled out in
mitodl/ol-infrastructure#4506.

The VERSION constant in Django settings is updated to the initial release
format version as part of this change.

How can this be tested?

1. With bumpver installed (pip install bumpver), run bumpver update --dry from the repo root and confirm it shows the expected diff with no errors.
2. Check that VERSION in Django settings matches current_version in pyproject.toml.

Additional Context

Part of the Concourse release pipeline modernization — migrating from the Doof
Slack bot to a Concourse-native release workflow using GitHub Issues as
production gates.

[github-actions]

OpenAPI Changes

Show/hide ## Changes for v0.yaml:

## Changes for v0.yaml:
No changes detected

## Changes for v1.yaml:
No changes detected

## Changes for v2.yaml:
No changes detected

Unexpected changes? Ensure your branch is up-to-date with main (consider rebasing).

[jkachel]

Sentry bot is correct - there's a test now failing because of the version format change. Otherwise LGTM; I was able to bump the version as expected.

[jkachel]

Tested with bump-my-version show-bump and that seemed to do things correctly, and the tests are now passing. So, LGTM 👍 ready to merge once the merge conflict is fixed.