feat: implement unified content engine to eliminate ID generation inconsistencies

- Create internal/engine module as single source of truth for content processing
- Consolidate 4 separate ID generation systems into one unified engine
- Update API handlers to use engine for consistent server-side ID generation
- Remove frontend client-side ID generation, delegate to server engine
- Ensure identical HTML markup + file path produces identical content IDs
- Resolve content persistence failures caused by ID fragmentation between manual editing and enhancement processes

internal/engine/markdown.go

@@ -0,0 +1,76 @@
+package engine
+
+import (
+	"bytes"
+	"log"
+	"strings"
+
+	"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 for inline content
+	// If content is wrapped in a single <p> tag, extract just the inner content
+	html = strings.TrimSpace(html)
+
+	if strings.HasPrefix(html, "<p>") && strings.HasSuffix(html, "</p>") {
+		// Check if this is a single paragraph (no other <p> tags inside)
+		inner := html[3 : len(html)-4] // Remove <p> and </p>
+		if !strings.Contains(inner, "<p>") {
+			// Single paragraph - return just the inner content for inline injection
+			return inner, nil
+		}
+	}
+
+	// Multiple paragraphs or other block content - return as-is
+	return html, nil
+}