feat: JWT auth, /api prefix, SPA serving, OpenAPI, lean main.rs

- auth: register/login/refresh/logout w/ JWT+Argon2, protected mutations
- domain: User, RefreshSession, auth ports, Unauthorized/Forbidden errors
- presentation: context/state/factory/errors/extractors/openapi modules
- routes behind /api, SPA served from root w/ fallback
- OpenAPI Scalar at /docs
- frontend ssr:false, single-binary Dockerfile

.dockerignore

@@ -1,5 +1,6 @@
 /target
-/app
+/app/node_modules
+/app/build
 .superpowers/
 .git/
 .claude/