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>

nextcloud_mcp_server/vector/html_processor.py

@@ -0,0 +1,49 @@
+"""HTML to Markdown conversion utilities for vector sync."""
+
+import logging
+
+from markdownify import markdownify as md
+
+logger = logging.getLogger(__name__)
+
+
+def html_to_markdown(html_content: str | None) -> str:
+    """Convert HTML content to Markdown, preserving semantic structure.
+
+    This function converts HTML (typically from RSS/Atom feed items) to Markdown
+    for better text embedding. Markdown preserves:
+    - Heading hierarchy (important for document structure)
+    - Lists (bullet and numbered)
+    - Links (as [text](url))
+    - Bold/italic emphasis
+    - Paragraphs and line breaks
+
+    Args:
+        html_content: HTML string to convert (may be None or empty)
+
+    Returns:
+        Markdown string, or empty string if input is None/empty
+
+    Example:
+        >>> html_to_markdown("<h1>Title</h1><p>Content with <b>bold</b>.</p>")
+        '# Title\\n\\nContent with **bold**.\\n\\n'
+    """
+    if not html_content:
+        return ""
+
+    try:
+        markdown = md(
+            html_content,
+            heading_style="ATX",  # Use # style headings
+            strip=["script", "style", "iframe", "noscript"],  # Remove unsafe elements
+            bullets="-",  # Use - for unordered lists
+            code_language="",  # Don't add language hints to code blocks
+        )
+        return markdown.strip()
+    except Exception as e:
+        logger.warning(f"Failed to convert HTML to Markdown: {e}")
+        # Fallback: strip all HTML tags as a last resort
+        import re
+
+        text = re.sub(r"<[^>]+>", " ", html_content)
+        return " ".join(text.split())  # Normalize whitespace