Use UTC for undated Atom entry timestamps

[shkyyy18]

Bug

For Atom entries without pubdate or updateddate, add_item_elements currently uses naive datetime.now(). rfc3339_date appends Z to naive values. On a UTC+8 machine at 20:34:56 local time, an undated entry therefore reports 20:34:56Z instead of the actual instant 12:34:56Z. The feed-level updated fallback already correctly uses an aware UTC clock.

Minimal fix

Use datetime.datetime.now(tz=datetime.timezone.utc) for the undated entry fallback, matching latest_post_date. Explicit publication/update dates and the public date-formatting function remain unchanged.

Validation

• Regression tests simulate UTC-5, UTC and UTC+8 host clocks without changing system timezone. They generate/parse a real Atom feed and compare entry and feed updated instants to the same fixed UTC clock.
• Before fix: 2 failed (non-UTC hosts), 1 passed (UTC control). After fix: all 3 pass.
• Python 3.12.10 / Windows: baseline 26 passed; after 29 passed, coverage 55% (includes bundled Django compatibility modules and tests, no coverage gate configured).
• Applicable changed-file pre-commit hooks passed except check-merge-conflict, blocked locally by Windows app-control WinError4551, and pyupgrade, which rewrites an unrelated existing RSS author-formatting line on both unchanged main and the patch. That baseline-only formatting change is deliberately not bundled in this one-line fix. No security policy changes made.
• Full multi-Python/platform CI matrix not run locally; upstream CI results remain separate.

Checked latest main, existing issues/PRs and repository guidance. AI-assisted investigation, implementation and synthetic test design; stated validation executed locally. No private data or unrelated refactoring.