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>

.gitea/sanitize-branch.sh

@@ -0,0 +1,11 @@
+#!/bin/sh
+# Shared branch slug sanitization for CI jobs.
+# Strips prefix (feature/, fix/, etc.), lowercases, replaces non-alphanum, truncates to 20 chars.
+sanitize_branch() {
+    echo "$1" | sed -E 's#^(feature|fix|feat|hotfix)/##' \
+      | tr '[:upper:]' '[:lower:]' \
+      | sed 's/[^a-z0-9-]/-/g' \
+      | sed 's/--*/-/g; s/^-//; s/-$//' \
+      | cut -c1-20 \
+      | sed 's/-$//'
+}