[Bug] Design list/get/export ignore SQLite-stored designs; README overstates SQLite as source of truth

[sjohnston1972]

Part of the "SQLite migration is half-done" parent issue. Do this last — after the build/journal sub-issues.

Background

The /design endpoints and the design list/export helpers still read the flat-file store, while orchestrator + legacy-imported designs live in SQLite section_designs.

Problem / Goal

• backend/app/api/design.py calls list_designs, get_design, delete_design from app.agents.loop — all flat-file (loop.py:43, 92, 100).
• Legacy-imported designs (legacy_import.py writes them to section_designs as section_name="FULL") and orchestrator designs are therefore invisible on GET /api/design/list and cannot be exported via /design/{id}/export.html|docx.
• The README claims SQLite covers "section designs" as the source of truth, which does not match these endpoints.

Goal: make the design read/list/export paths surface SQLite-stored designs, and correct the README so its persistence claims are accurate.

Where to look

• backend/app/api/design.py:79-142 — /list, /{id}, delete, export.html, export.docx.
• backend/app/agents/loop.py:43-111 — flat-file list_designs, get_design, delete_design.
• backend/app/db/models.py:82 — SectionDesign (record, legacy_id, final_verdict, etc.).
• backend/app/export/render.py — to_html / to_docx consume the design record dict; confirm the SQLite record is the same shape (it is — legacy_import.py copies it verbatim).
• README.md — persistence section + "Files of interest".

Suggested approach

1. Point list_designs/get_design/delete_design (or new DB-backed equivalents used by api/design.py) at section_designs. A design "id" becomes the SectionDesign.id; record is the payload the renderers already expect.
2. Keep the /design/list response fields the same (summary, decisions, citations, composition, outstanding_flaws) — they're all derivable from record["iterations"][-1].
3. Export endpoints just need get_design to return the SQLite record.
4. Update README.md so the persistence claims (source of truth, journal in SQLite) are true after the whole parent lands.

Acceptance criteria

• GET /api/design/list includes orchestrator-run and legacy-imported designs.
• GET /api/design/{id} and the HTML/DOCX exports work for a SQLite-stored design.
• README's persistence statements match the code.

Testing

Local backend. Import a legacy design (or produce one via a run), then:

• GET /api/design/list shows it.
• GET /api/design/{id}/export.html returns a self-contained HTML doc.
  Add a unit test that a SectionDesign.record round-trips through export_render.to_html without error.

Out of scope

• Any change to the record JSON shape.
• Deduping the (now overlapping) legacy flat-file /designs directory — it can remain unused on disk.