Synthesis failure UX: error-state storage + regeneration workflows

[charlie83Gs]

Context

Originally drafted as PR #58 (closed). The PR was based on v0.7.1 and conflicts heavily with the current synthesizer architecture (ReadGraphEngine/WorkerGraphEngine split, model_id_override, normalize_ai_links, resolve_sessions/graph_id). Closing and re-opening as a narrower, additive change against current main.

Problem

When the synthesizer or super-synthesizer agent ends without producing text, the workflow currently stores a placeholder string ("Synthesis completed but no document was produced.") as the node's definition. That masks failure — users see a junk document instead of a clear "this failed, retry it" affordance.

Proposed behaviour

Backend — worker-synthesis

• On empty agent output, do NOT set a definition. Instead store:
  • metadata_.synthesis_error = {message, timestamp}
  • metadata_.synthesis_input = <original SynthesizerInput dump> (for regeneration)
• On success: clear synthesis_error, persist synthesis_input.
• Super-synthesizer: stamp parent_supersynthesis_id onto each sub-synthesis so sub regeneration can cascade a recombine.

New workflows

• regenerate_synthesis_wf(node_id) — re-runs the synthesizer agent on a failed synthesis node in-place (preserves node ID). If it has a parent super-synthesis, dispatches recombine_supersynthesis_wf on completion.
• recombine_supersynthesis_wf(node_id) — re-runs only the combine step on an existing super-synthesis using its current sub-synthesis documents.

Register both in worker-all.

API — services/api

• Add status: \"completed\" | \"error\" + error_message: str | null to SynthesisDocumentResponse and SynthesisListItem.
• POST /api/v1/syntheses/{id}/regenerate — validates node is in error state, dispatches the right regeneration workflow via dispatch_with_graph.

Hatchet I/O models — libs/kt-hatchet

• RegenerateSynthesisInput(GraphAwareMixin) { node_id: str }
• RecombineSuperSynthesisInput(GraphAwareMixin) { node_id: str }

Frontend

• Show red "Failed" badge on failed syntheses in the investigate list.
• In the synthesis document view, replace the body with an error banner + "Regenerate" button when status === \"error\".
• regenerateSynthesis(id) helper in lib/api.ts (uses graphRequest).
• Optional: poll for regeneration progress.

Scope notes

• Keep it additive. No rewrite of the existing synthesizer/super_synthesizer bodies beyond swapping the placeholder-text branch for the error-state storage.
• The PR feat(worker-synthesis): store synthesis errors and add regeneration #58 draft (branch feat/synthesis-error-state-regenerate, commit 98d7a3e) has working reference code — but its _helpers.py was designed around the old GraphEngine. Port the error-state pieces onto the current ReadGraphEngine + WorkerGraphEngine pattern.

Links

• Closed PR: feat(worker-synthesis): store synthesis errors and add regeneration #58
• Reference commit: 98d7a3e on feat/synthesis-error-state-regenerate