feat: implement minimal server-first markdown processing

Backend implementation:
- Add goldmark dependency for markdown processing
- Create MarkdownProcessor with minimal config (bold, italic, links only)
- Update content injector with HTML injection capabilities
- Add injectHTMLContent() for safe DOM manipulation
- Server now converts **bold**, *italic*, [links](url) to HTML during enhancement

Frontend alignment:
- Restrict marked.js to match server capabilities
- Disable unsupported features (headings, lists, code blocks, tables)
- Update turndown rules to prevent unsupported markdown generation
- Frontend editor preview now matches server output exactly

Server as source of truth:
- Build-time markdown→HTML conversion during enhancement
- Zero runtime overhead for end users
- Consistent formatting between editor preview and final output
- Raw markdown stored in database, HTML served to visitors

Tested features:
- **bold** → <strong>bold</strong> ✅
- *italic* → <em>italic</em> ✅
- [text](url) → <a href="url">text</a> ✅

internal/content/enhancer.go

@@ -108,7 +108,7 @@ func (e *Enhancer) findAndInjectNodes(rootNode *html.Node, elem parser.Element,
 	}
 
 	// Inject content attributes for the correctly matched node
-	e.injector.addContentAttributes(targetNode, elem.ContentID, string(elem.Type))
+	e.injector.AddContentAttributes(targetNode, elem.ContentID, string(elem.Type))
 
 	// Inject content if available
 	if contentItem != nil {