feat(dynamodb): optimistic concurrency on update() (#24)

[R3108]

update() now stores a version attribute on the DynamoDB item and writes it with a ConditionExpression, so a concurrent change raises ConflictError (nothing written) instead of being silently overwritten.

• DynamoDBStore.put_versioned: conditional put_item that bumps the version; ConditionalCheckFailedException -> ConflictError
• DynamoDBStore.get_versioned: strongly consistent GetItem incl. version
• Dynavec.update: optional expected_version=, returns the new version on UpsertResult.version; writes DynamoDB before S3 Vectors so a conflict leaves both stores untouched
• Plain upsert() stays last-writer-wins and drops the version
• moto-backed store tests + in-memory client tests for conflicts

Description

Related issue

Fixes #

Changes

Testing

• Tests pass locally
• Ruff checks pass
• Documentation updated, if applicable

Checklist

• My changes are focused and relevant to this pull request.
• I have added or updated tests where appropriate.
• I have reviewed my changes for unrelated modifications.
• I have updated documentation where necessary.

[Kaap10]

Hey @R3108, the code looks great! Could you please clean up the PR description template and add Closes #24 so GitHub links and auto-closes the issue upon merge?

[codeforstartups]

Excellent — real optimistic concurrency for update() (#24). Each item carries a version, and updates write with a ConditionExpression so a concurrent change raises ConflictError (nothing written) instead of a silent lost update. Nice touches: expected_version= to guard across your own read/modify/write cycle, UpsertResult.version returning the new version, and upsert() deliberately staying last-writer-wins. Verified locally: ruff clean, 58 tests pass, CI green. Docs + CHANGELOG updated. Merging — thanks @R3108! 🙌