Fix paragraph handler losing inline formatting (bold, italic, links)

MarkdownConverter correctly parses markdown into a rich AST, but
_add_blocks_to_post was calling _extract_text_from_content() which
re-serialized the AST back to a plain string — causing literal asterisks
to appear in Substack instead of bold/italic text.

Pass content directly to post.paragraph() since the Post class already
supports rich content lists natively.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: johnnyoshika

src/handlers/post_handler.py

@@ -1287,9 +1287,8 @@ def _add_blocks_to_post(self, post: Post, blocks: List[Dict[str, Any]]):
             content = block.get("content")
 
             if block_type == "paragraph":
-                # Extract text and add as single paragraph
-                text_content = self._extract_text_from_content(content)
-                post.paragraph(text_content)
+                # Pass content list directly to preserve inline formatting (bold, italic, links)
+                post.paragraph(content)
 
             elif block_type in [
                 "heading-one",