LLM-chain: database schema for chain execution tracking

[vprashrex]

Is your feature request related to a problem? Please describe.
Chain execution state is spread across job and individual llm_call records, no single source for chain-level progress, checkpointing, or debugging without joining multiple tables.

Describe the solution you'd like
A dedicated llm_chain table that acts as a workflow manager, tracks chain progress, adding checkpoints so the entire llm-execution is visible from a single row.

Describe alternatives you've considered
Reference how Airflow tracks per-task state and how Temporal stores workflow execution history with per-activity checkpoints.

Additional context
Schema should persist that block-level state to the database for queryability and potential resume support.