feat: add admin publishing workflow and yar theme

Add Go/Postgres admin APIs, Angular admin UI, manual build flow, asset uploads, markdown import/export, configurable slug generation, and the Yar reading theme. Exclude local docs and generated development artifacts from version control.

backend/internal/admin/static.go

@@ -0,0 +1,51 @@
+package admin
+
+import (
+	"embed"
+	"net/http"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"github.com/gin-gonic/gin"
+)
+
+//go:embed web/*
+var adminWeb embed.FS
+
+func adminIndex(adminDir string) ([]byte, error) {
+	if strings.TrimSpace(adminDir) != "" {
+		path := filepath.Join(adminDir, "index.html")
+		if page, err := os.ReadFile(path); err == nil {
+			return rewriteAdminBase(page), nil
+		}
+	}
+	return adminWeb.ReadFile("web/index.html")
+}
+
+func serveAdminFile(c *gin.Context, adminDir string) bool {
+	if strings.TrimSpace(adminDir) == "" {
+		return false
+	}
+
+	requested := strings.TrimPrefix(c.Param("filepath"), "/")
+	if requested == "" {
+		return false
+	}
+	cleaned := filepath.Clean(requested)
+	if cleaned == "." || strings.HasPrefix(cleaned, "..") {
+		return false
+	}
+
+	path := filepath.Join(adminDir, cleaned)
+	info, err := os.Stat(path)
+	if err != nil || info.IsDir() {
+		return false
+	}
+	http.ServeFile(c.Writer, c.Request, path)
+	return true
+}
+
+func rewriteAdminBase(page []byte) []byte {
+	return []byte(strings.Replace(string(page), `<base href="/">`, `<base href="/admin/">`, 1))
+}