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/database.go

@@ -0,0 +1,20 @@
+package admin
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/jackc/pgx/v5/pgxpool"
+)
+
+func OpenDatabase(ctx context.Context, databaseURL string) (*pgxpool.Pool, error) {
+	pool, err := pgxpool.New(ctx, databaseURL)
+	if err != nil {
+		return nil, fmt.Errorf("create postgres pool: %w", err)
+	}
+	if err := pool.Ping(ctx); err != nil {
+		pool.Close()
+		return nil, fmt.Errorf("ping postgres: %w", err)
+	}
+	return pool, nil
+}