feat: add atomic changeset application core

[yuyukosama2004]

Summary

• add ChangeSet and ChangeOperation persistence with Alembic migration 20260718_0003
• implement block-level insert_before, insert_after, replace_block, and delete_block
• enforce baseline document hashes, stable node IDs, original hashes, orphan/conflict outcomes, partial decisions, and safe rebase
• expose create/list/get/apply APIs with optimistic draft revision checks and one-transaction draft/status updates
• keep SceneVersion and Canon history immutable; accepted operations only create/update the working draft

Verification

• backend: Ruff, mypy, 96 pytest tests
• frontend regression: ESLint, 62 Vitest tests, production build (unchanged frontend)
• migration upgrade/downgrade/re-upgrade included in the full suite
• API tests cover partial accept, stale revision rejection, safe rebase, all-reject no-op, and stale baseline hash rejection