T-5: set up API-first tooling with OpenAPI spec as single source of truth

Backend: openapi-generator-maven-plugin generates Spring interfaces and DTOs
from the spec. Frontend: openapi-typescript + openapi-fetch provide type-safe
API access. Both sides get compile-time contract enforcement from a single
api.yaml file.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -39,6 +39,7 @@ replay_pid*
 node_modules/
 dist/
 build/
+frontend/src/api/schema.d.ts
 vite.config.js.timestamp-*
 vite.config.ts.timestamp-*
 npm-debug.log*