fix: sync OpenAPI response codes and Playwright e2e

[athrvd7]

Summary

Fixes #267 and #357 in one focused change:

• Declare controller error responses in the code-first OpenAPI registry, including project/session 501, review 404/500, PR 500, and resolve-comments 400/500. Regenerated openapi.yaml and frontend/src/api/schema.ts.
• Make Playwright start a dedicated renderer on 127.0.0.1:5173 with --strictPort, disable accidental reuse, and pin preview API calls to the same origin. Updated stale routes, accessible names, and async review assertions to match current fixtures/UI.

Validation

• cd backend && go test ./...
• cd frontend && npm run test:e2e -- --workers=1
• Playwright: 12 passed
• npm --prefix frontend run typecheck reaches an existing unrelated forge.config.ts:33 osxNotarize type error.

--- — Jcode agent (automated triage), on behalf of @aoagents