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/builder.go

@@ -0,0 +1,91 @@
+package admin
+
+import (
+	"bytes"
+	"context"
+	"fmt"
+	"os/exec"
+	"sync"
+	"time"
+)
+
+type Builder struct {
+	store    *Store
+	exporter *Exporter
+	siteDir  string
+	queue    chan string
+	once     sync.Once
+}
+
+func NewBuilder(store *Store, exporter *Exporter, siteDir string) *Builder {
+	return &Builder{
+		store:    store,
+		exporter: exporter,
+		siteDir:  siteDir,
+		queue:    make(chan string, 32),
+	}
+}
+
+func (b *Builder) Start(ctx context.Context) {
+	b.once.Do(func() {
+		go b.loop(ctx)
+	})
+}
+
+func (b *Builder) Enqueue(jobID string) bool {
+	select {
+	case b.queue <- jobID:
+		return true
+	default:
+		return false
+	}
+}
+
+func (b *Builder) loop(ctx context.Context) {
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case jobID := <-b.queue:
+			b.runBuildJob(ctx, jobID)
+		}
+	}
+}
+
+func (b *Builder) runBuildJob(ctx context.Context, jobID string) {
+	log, err := b.run(ctx, jobID)
+	if err != nil {
+		_ = b.store.MarkBuildJobFailed(context.Background(), jobID, log, err.Error())
+		return
+	}
+	_ = b.store.MarkBuildJobSuccess(context.Background(), jobID, log)
+}
+
+func (b *Builder) run(ctx context.Context, jobID string) (string, error) {
+	if err := b.store.MarkBuildJobRunning(ctx, jobID); err != nil {
+		return "", err
+	}
+
+	posts, err := b.store.PublishedPostsForExport(ctx)
+	if err != nil {
+		return "", err
+	}
+	if err := b.exporter.ExportPublishedPosts(ctx, posts); err != nil {
+		return "", err
+	}
+
+	buildCtx, cancel := context.WithTimeout(ctx, 2*time.Minute)
+	defer cancel()
+
+	cmd := exec.CommandContext(buildCtx, "npm", "run", "build")
+	cmd.Dir = b.siteDir
+
+	var output bytes.Buffer
+	cmd.Stdout = &output
+	cmd.Stderr = &output
+	if err := cmd.Run(); err != nil {
+		return output.String(), fmt.Errorf("astro build failed: %w", err)
+	}
+
+	return output.String(), nil
+}