fix(og): preserve unchanged generated images

[TimeToBuildBob]

Summary

• add pixel-equality save guard for generated OG images
• make post/default generators return whether they actually wrote files
• add regression tests for preserving mtimes on identical output

Verification

• uv run --with pillow,pyyaml,pytest pytest tests/test_generate_og_images.py -q
• uv run --with pillow,pyyaml python3 scripts/generate_og_images.py after touching a tracked post: 0 generated, 404 skipped, existing OG mtime unchanged