Evaluation: Automate prompt improvement #1159

Problem

Improving a config's prompt today is a manual loop: run an eval, read scores, call improve-prompt, wait, kick off another eval, repeat. There's no way to hand the whole cycle to Kaapi and get back a report once it converges.

Proposal

Add POST /api/v2/evaluations/iterations to run a self-driving loop that chains fast-eval and v2 prompt improvement via a LangGraph state machine:

start_eval → wait_eval → (conditional) start_improve → wait_improve → loop back to start_eval, or → finalize.

• Stop score = mean of Adherence to Ground Truth + Adherence to Prompt; Adherence to Knowledge Base is tracked per round for visibility only and never gates stopping.
• Stops on 3 consecutive rounds under the ceiling-delta threshold (ceiling_reached), a max_rounds cap (max_rounds_reached), or a hard round failure (round_failed).
• Round-by-round state lives in a Postgres LangGraph checkpoint (thread_id = str(iteration_run.id)); the evaluation_iteration_run table is thin bookkeeping only (status/stop_reason/dataset/config/callback).
• Resumption piggybacks on the existing /cron/evaluations tick rather than a new scheduler — no orchestrator polls a provider directly.
• Final round-by-round report + best round delivered to the caller's callback_url, same best-effort convention as prompt-improvement callbacks.

Notes

Full design detail: docs/wiki/modules/evaluations.md (Async section) and docs/architecture/kaapi-evaluations-ARCHITECTURE.md.