Generate the frontend API client from qh's OpenAPI spec

[thorwhalen]

Summary

frontend/src/api/schema.ts was hand-written and verified against the live backend, because qh's /openapi.json emitted empty request/response schemas. With i2mint/qh#9, qh now derives full JSON Schema from ef.service.EfService's Python type hints — so the frontend's API types are now generated from the spec.

What changed

• backend/export_openapi.py — exports the backend's OpenAPI document to frontend/src/api/openapi.json, the committed API-contract snapshot.
• Frontend tooling — openapi-typescript devDependency + a gen:api script that regenerates src/api/openapi.d.ts from openapi.json.
• src/api/schema.ts — was 113 lines of hand-written interfaces; now thin aliases (CorpusInfo, Segment, SearchHit, ExploreResult, CreateCorpusBody, QueryBody, ExploreBody) over the generated types. All 8 consumer files are untouched — the named-alias layer keeps the import surface stable.
• src/surfaces/SearchSurface.tsx — guards SearchHit.source_id. The generated types correctly show it as nullable (ef's source_id: str | None); the hand-written schema had wrongly declared it always-present, so this is a latent-bug fix the generated types surfaced.
• .gitignore — track frontend/src/api/openapi.json (it was caught by a broad *.json ignore).

Refresh workflow

After a backend API change:

cd backend  && python export_openapi.py   # refresh openapi.json
cd frontend && pnpm gen:api               # regenerate openapi.d.ts

Verification

• pnpm typecheck and pnpm build pass.
• Backend serves the enriched /openapi.json (with components.schemas).
• create_corpus → search → explore_corpus → list_corpora → delete_corpus all work over HTTP.
• A browser round-trip through the UI — create a corpus via the form, then search — renders results with zero console/page errors.

Closes #7
Refs i2mint/qh#9