fix(retain): preserve document created_at across upsert + UI edit flow

[nicoloboschi]

Summary

• Fix: re-ingesting a document via retain with the same document_id used to reset created_at to NOW() because the explicit DELETE on the documents row ran before the INSERT ... ON CONFLICT DO UPDATE — so the ON CONFLICT branch was never reached. Now we RETURNING created_at on the DELETE and pass it through to the INSERT (COALESCE($7, NOW())), preserving the original creation timestamp. updated_at still advances on every write.
• UI: file upload defaults document_id to the file name instead of letting the server generate a UUID.
• UI: documents table gains an Updated column alongside Created.
• UI: document detail panel supports editing original_text. Save calls retain with the same document_id plus the original context, event_date, metadata, and tags (preserved from retain_params), triggering the upsert path — which now correctly keeps created_at.

Test plan

• uv run pytest tests/test_retain.py::test_document_upsert_preserves_created_at — new regression test (passes)
• uv run pytest tests/test_retain.py::test_document_upsert_behavior tests/test_delta_retain_duplicates.py — existing upsert + concurrent-upsert suite (5 passed)
• ./scripts/hooks/lint.sh — green
• Manual: upload a file via the Add Document → Upload tab; confirm document_id defaults to the file name
• Manual: open a document, edit its content, save; confirm updated_at advances while created_at stays stable, and tags/metadata/context are preserved