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/markdown.go

@@ -0,0 +1,67 @@
+package content
+
+import (
+	"bytes"
+	"log"
+
+	"github.com/yuin/goldmark"
+	"github.com/yuin/goldmark/parser"
+	"github.com/yuin/goldmark/renderer/html"
+	"github.com/yuin/goldmark/util"
+)
+
+// MarkdownProcessor handles minimal markdown processing
+// Supports only: **bold**, *italic*, and [link](url)
+type MarkdownProcessor struct {
+	parser goldmark.Markdown
+}
+
+// NewMarkdownProcessor creates a new markdown processor with minimal configuration
+func NewMarkdownProcessor() *MarkdownProcessor {
+	// Configure goldmark to only support basic inline formatting
+	md := goldmark.New(
+		goldmark.WithParserOptions(
+			parser.WithInlineParsers(
+				// Bold (**text**) and italic (*text*) - same parser handles both
+				util.Prioritized(parser.NewEmphasisParser(), 500),
+
+				// Links [text](url)
+				util.Prioritized(parser.NewLinkParser(), 600),
+			),
+			// Disable all block parsers except paragraph (no headings, lists, etc.)
+			parser.WithBlockParsers(
+				util.Prioritized(parser.NewParagraphParser(), 200),
+			),
+		),
+		goldmark.WithRendererOptions(
+			html.WithXHTML(),     // <br /> instead of <br>
+			html.WithHardWraps(), // Line breaks become <br />
+			html.WithUnsafe(),    // Allow existing HTML to pass through
+		),
+	)
+
+	return &MarkdownProcessor{parser: md}
+}
+
+// ToHTML converts markdown string to HTML
+func (mp *MarkdownProcessor) ToHTML(markdown string) (string, error) {
+	if markdown == "" {
+		return "", nil
+	}
+
+	var buf bytes.Buffer
+	if err := mp.parser.Convert([]byte(markdown), &buf); err != nil {
+		log.Printf("Markdown conversion failed: %v", err)
+		return "", err
+	}
+
+	html := buf.String()
+
+	// Clean up goldmark's paragraph wrapping - we want inline content
+	// Remove <p> and </p> tags if the content is wrapped in a single paragraph
+	if len(html) > 7 && html[:3] == "<p>" && html[len(html)-4:] == "</p>" {
+		html = html[3 : len(html)-4]
+	}
+
+	return html, nil
+}