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> ✅

go.mod

@@ -26,6 +26,7 @@ require (
 	github.com/spf13/cast v1.6.0 // indirect
 	github.com/spf13/pflag v1.0.5 // indirect
 	github.com/subosito/gotenv v1.6.0 // indirect
+	github.com/yuin/goldmark v1.7.8 // indirect
 	go.uber.org/atomic v1.9.0 // indirect
 	go.uber.org/multierr v1.9.0 // indirect
 	golang.org/x/exp v0.0.0-20230905200255-921286631fa9 // indirect