feat: add durable workflow runtime core

[yuyukosama2004]

R2 durable runtime core

• add normalized workflow step attempts and durable ordered events
• add idempotent enqueue and an atomic SQLite-safe lease claim
• fence expired workers with owner and lease checks
• commit raw model output before parsing, then persist output hashes and checkpoints
• requeue retryable failures as a new workflow and step attempt
• preserve legacy workflow rows across upgrade/downgrade/re-upgrade

Verification

• Ruff check and format: passed
• mypy (93 source files): passed
• focused runtime/migration tests: 6 passed
• full backend suite: 83 passed
• Alembic base/head/check/downgrade/re-upgrade: passed