feat: article version history with abstract/title diff

[xenacode-art]

Closes #159

Adds version tracking for articles so users can see what changed between edits (title and abstract diffs).

Changes

articles/models.py

• New ArticleVersion model — stores title, abstract, version number, and created_at for each edit

articles/api.py

• update_article() now snapshots the current title+abstract into ArticleVersion before overwriting
• New endpoint: GET /articles/{article_id}/versions — returns a list of versions, each with a unified diff for title and abstract
  (uses Python stdlib difflib)

articles/schemas.py

• New ArticleVersionDiffOut schema with version, created_at, title_diff: List[str], abstract_diff: List[str]

articles/migrations/0034_articleversion.py

• Migration to create the ArticleVersion table

Example response

[
  {
    "version": 1,
    "created_at": "2026-03-26T10:00:00Z",
    "title_diff": ["--- ", "+++ ", "@@ -1 +1 @@", "-Old Title", "+New Title"],
    "abstract_diff": ["--- ", "+++ ", "@@ -1,2 +1,2 @@", "-Old abstract text.", "+Updated abstract text."]
  }
]

Test plan

- Edit an article via PUT /articles/{id} and confirm an ArticleVersion row is created
- Call GET /articles/{id}/versions and verify the diff output is correct
- Confirm articles with no edits return []
- Confirm 404 is returned for unknown article IDs