refactor: architecture cleanup — OIDC dedup, PKCE, K8s hardening

- Extract OidcProviderHelper for shared discovery + JWK source construction
- Add SystemRole.normalizeScope() to centralize role normalization
- Merge duplicate claim extraction in OidcTokenExchanger
- Add PKCE (S256) to OIDC authorization flow (frontend + backend)
- Add SecurityContext (runAsNonRoot) to all K8s deployments
- Fix postgres probe to use $POSTGRES_USER instead of hardcoded username
- Remove default credentials from Dockerfile
- Extract sanitize_branch() to shared .gitea/sanitize-branch.sh
- Fix sidebar to use /exchanges/ paths directly, remove legacy redirects
- Centralize basePath computation in router.tsx via config module

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Dockerfile

@@ -18,10 +18,6 @@ FROM eclipse-temurin:17-jre
 WORKDIR /app
 COPY --from=build /build/cameleer3-server-app/target/cameleer3-server-app-*.jar /app/server.jar
 
-ENV SPRING_DATASOURCE_URL=jdbc:postgresql://postgres:5432/cameleer3
-ENV SPRING_DATASOURCE_USERNAME=cameleer
-ENV SPRING_DATASOURCE_PASSWORD=cameleer_dev
-
 EXPOSE 8081
 ENV TZ=UTC
 ENTRYPOINT exec java -Duser.timezone=UTC -jar /app/server.jar