feat: add stable document node identities

[yuyukosama2004]

Summary

• upgrade newly saved scene documents to novelflow.tiptap.v2 with stable UUIDs on addressable block nodes
• preserve frozen v1 history and Canon hashes; no historical document rewrite or database migration
• repair missing, invalid, and duplicate IDs deterministically on the backend
• assign IDs to new Tiptap nodes in the editor and load authoritative content_json instead of rebuilding from Markdown

Verification

• backend: Ruff, mypy, 88 pytest tests
• frontend: ESLint, 62 Vitest tests, production build
• explicit tests for deterministic repair, duplicate handling, new-node identity, and version-switch identity preservation