feat: run scene generation in durable worker

[yuyukosama2004]

Summary

• move scene generation out of the request lifecycle into a FastAPI lifespan database worker
• persist planning, drafting, summary, raw model output, event cursors, and version linkage for crash recovery
• resume SSE with Last-Event-ID and keep client retries idempotent
• add lease fencing, cancellation notification, retry, migration, and forced-crash coverage

Verification

• backend: Ruff, mypy, 87 pytest tests
• frontend: ESLint, 61 Vitest tests, production build
• repeated partial-stream terminal race test: 10/10 passed
• Alembic upgrade/downgrade/re-upgrade covered by migration test