Add multiple photos to pet social posts

[Smoss]

Summary

• Fetch up to four distinct, shelter-ordered RescueGroups photos per pet, with the existing thumbnail fallback.
• Publish usable photos in one Mastodon or Bluesky post, and use an Instagram carousel when at least two photos are ready.
• Preserve single-photo callers and add per-photo alt text, debug output, and partial-failure handling.

Tests

• .venv/bin/python -m pytest -q — 145 passed, 1 skipped.

Closes #169