First Phase 2 site migration. _save_to_library shrinks from ~80 lines
to ~30 by delegating to core.library_save.save_post_file. The
"find existing copy and rename across folders" block is gone — same-
post idempotency is now handled by the DB-backed filename column via
_same_post_on_disk inside save_post_file. The thumbnail-copy block is
extracted as a new _copy_library_thumb helper so _bulk_save (Phase
2.2) can call it too.
behavior change from v0.2.3: cross-folder re-save is now copy, not
move. Old folder's copy is preserved. The atomic-rename-move was a
workaround for not having a DB-backed filename column; with
_same_post_on_disk the workaround is unnecessary. Users who want
move semantics can manually delete the old copy.
Net diff: -52 lines.
pax
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