Add 3.0.0a2 changelog; restore 3.0.0a1 to tag-time content

- Insert new [3.0.0a2] section with changes since the a1 tag:
  rate-limit handling, lazy imports, is-not-defined workarounds,
  objects.py removal, config parse errors raising, icalendar-searcher
  bump, fallbacks, bug fixes, compatibility hints updates
- Restore [3.0.0a1] section to its exact content at the v3.0.0a1 tag
  (the previous commit had retroactively added post-a1 items to it)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: tobixen