feat: content fetch, sitemap fixes, remove oidcAuthFlow

- Add contentFetchFlow() to proxy (FR-001 through FR-012)
- Add extractArticleBody() helper with vkm:articleBody / articleBody fallback
- Dynamic proxyBaseUrl derivation from x-forwarded-proto/host headers
- Forward query/size/category params on /sitemap.xml requests
- Add Accept: application/ld+json header to content API calls
- Remove oidcAuthFlow() - unmatched requests now return 404 Not Found
- Fix xmlbuilder2 import: default import, call as xmlbuilder2.create(...)
- Version bump 0.2.0 → 0.3.0
- 45/45 tests passing

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

specs/002-sitemap-generation/data-model.md

@@ -156,7 +156,7 @@ Incoming GET /…/sitemap.xml
   (skip empty vkm:url)
          |
          v
-  Build SitemapDocument (xmlBuilder)
+  Build SitemapDocument (xmlbuilder2)
          |
          v
   200 OK