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/migrations_test.go

@@ -0,0 +1,35 @@
+package admin
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestLoadMigrationFiles(t *testing.T) {
+	dir := t.TempDir()
+	files := map[string]string{
+		"002_second.sql": "select 2;",
+		"001_first.sql":  "select 1;",
+		"notes.txt":      "ignored",
+	}
+	for name, content := range files {
+		if err := os.WriteFile(filepath.Join(dir, name), []byte(content), 0o600); err != nil {
+			t.Fatalf("write fixture: %v", err)
+		}
+	}
+
+	migrations, err := LoadMigrationFiles(dir)
+	if err != nil {
+		t.Fatalf("load migrations: %v", err)
+	}
+	if len(migrations) != 2 {
+		t.Fatalf("expected 2 migrations, got %d", len(migrations))
+	}
+	if migrations[0].Version != "001_first.sql" || migrations[1].Version != "002_second.sql" {
+		t.Fatalf("unexpected migration order: %#v", migrations)
+	}
+	if migrations[0].Checksum == "" || migrations[0].Checksum == migrations[1].Checksum {
+		t.Fatalf("unexpected checksums: %#v", migrations)
+	}
+}