Add OAuth2/OIDC per-user Gitea authentication

Introduce a GiteaOAuthValidator for JWT and userinfo validation and
fallbacks, add /oauth/token proxy, and thread per-user tokens through
the
request context and automation paths. Update config and .env.example for
OAuth-first mode, add OpenAPI, extensive unit/integration tests,
GitHub/Gitea CI workflows, docs, and lint/test enforcement (>=80% cov).

requirements.txt

@@ -5,4 +5,6 @@ pydantic>=2.5.0
 pydantic-settings>=2.1.0
 PyYAML>=6.0.1
 python-dotenv>=1.0.0
+python-multipart>=0.0.9
 structlog>=24.1.0
+PyJWT[crypto]>=2.9.0