feat(api): REST API with OpenAPI for other apps, behind Pireon OAuth

A REST adapter next to GraphQL (presenters/http) that calls the same
application services, so validation, permissions and domain errors are the
same: projects (members, board, stats, activity), issues (search, workflow,
assignee, release, links, comments, history), comments, releases and users.
Projects and issues can be addressed by id or key (WEB, WEB-12).

- Swagger UI at /api/docs, document at /api/docs-json and apps/api/openapi.json
  (rewritten by `nx serve api`, like schema.gql) for client generation.
- Every route but health/auth/webhook needs a Pireon token: the document
  declares Pireon OAuth2 (authorization code + PKCE, password) through the
  /api/auth/token proxy, and a bearer scheme for apps with their own client.
- configureApp() shares main.ts setup with the e2e, which now also covers REST.
- SWAGGER_ENABLED / OPENAPI_FILE config; docs 03/05 and library READMEs.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.env.example

@@ -28,6 +28,10 @@ AUTH_SESSION_TTL_SECONDS=60
 
 GRAPHQL_PLAYGROUND=true
 
+# REST + OpenAPI for other apps (Swagger UI at /api/docs). The document is also written to OPENAPI_FILE.
+SWAGGER_ENABLED=true
+OPENAPI_FILE=apps/api/openapi.json
+
 # Docker builds only: token of Pireon's Nexus for @pi/ui (build secret, never stored in an image).
 # Locally: export NEXUS_TOKEN=$(grep '^//nexus.pireon.dev/repository/npm-releases/:_authToken=' ~/.npmrc | cut -d= -f2-)
 NEXUS_TOKEN=