Adds a five-segment progress model for the document parsing pipeline so
the UI (PR③) can render a timeline showing where each document is
(DocReader → Chunking → Embedding → Multimodal → PostProcess) and
which stage failed with what error code.
- New table `knowledge_processing_stages` (migration 000052) with one
row per (knowledge_id, stage). UPSERT on Begin/Done/Fail bumps an
attempt counter so re-parses don't lose history.
- StageTracker service exposes Begin/Done/Fail/Skip; all calls are
best-effort and never break the pipeline if persistence fails.
- Stable error codes (DOCREADER_TIMEOUT / EMBEDDING_RATE_LIMIT /
VECTORSTORE_WRITE_FAILED / ...) the UI can map to localized
remediation hints.
- Tracker call sites added at the four meaningful failure points:
convert (DocReader), CreateChunks (Chunking), BatchIndex (Embedding),
enqueueImageMultimodalTasks (Multimodal start),
KnowledgePostProcess.Handle (Multimodal close + PostProcess).
- New endpoint `GET /api/v1/knowledge/:id/stages` returns the five
canonical stages — missing rows are synthesized as "pending" so
the timeline always renders five segments. Includes current_stage
and last_error block.
