feat(routes): let add_routes leave response bodies out of /cached-records

[allen0099]

Closes #79.

Change

add_routes() gains include_content_preview: bool = True. When it is False, every record in /cached-records has content_preview: null. Keys, sizes, ETags and expiry are still reported, and /cached-hits is unchanged (it never included bodies).

• The default stays True, so existing callers see no change. Whether it should flip to False in 0.4.0 remains the open question in the issue.
• The CachedRecord.content_preview field type widens from str to str | None.
• docs/HTTP_CACHING.md shows the option in the example and mentions it in the route description and the no-authentication warning. [Unreleased] has an Added entry.

Tests

• New test_cached_records_can_omit_content_preview:
  • The preview is null, and the body string appears nowhere in the response.
  • Path, size and ttl are still reported.
• Local results:
  • Full suite against the throwaway Redis (6380) and Memcached (11212) test servers: 690 passed, 1 skipped, coverage 100%.
  • ruff, mypy --strict and zensical build --strict are clean.

Note

This PR and #94 both edit [Unreleased] in CHANGELOG.md: #94 adds a ### Fixed section and this PR adds to ### Added. The edits are in different places, so they should merge cleanly.