feat(news): add Nextcloud News app integration

Add full integration for the Nextcloud News (RSS/Atom reader) app:

- Add NewsClient with complete CRUD operations for folders, feeds, and items
- Add 8 read-only MCP tools for listing/getting folders, feeds, items
- Add Pydantic models for News entities with camelCase alias support
- Add vector sync support for starred + unread items
- Add HTML to Markdown converter using markdownify for better embeddings
- Add Docker post-install hook to enable News app
- Add 25 unit tests for NewsClient API methods

Vector sync indexes starred and unread items, providing a balanced approach
that captures important (starred) and current (unread) content without
indexing the entire article history.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

pyproject.toml

@@ -36,6 +36,7 @@ dependencies = [
     "python-json-logger>=3.2.0", # Structured JSON logging
     "jinja2>=3.1.6",
     "langchain-text-splitters>=1.0.0",
+    "markdownify>=0.14.1",  # HTML to Markdown conversion for News items
     "pymupdf>=1.26.6",
     "pymupdf4llm>=0.2.2",
     "pymupdf-layout>=1.26.6",