feat: robots.txt served file, AI bots and the non-production lock

- github.com/jimsmart/grobotstxt v1.0.3 (D-99, decision note in the
  framework repo)
- golem15_seo_robots singleton: rules and AI bot switches; eleven bots
  incl. Claude-User, all allowed by default
- GET /robots.txt: rules, a Disallow group per switched-off bot and the
  golem15.seo.robots_sitemaps lines; outside production the lock with
  X-Robots-Tag; never a 5xx (last good body, else allow-all)
- TestRobots on Google's matcher with the deciding line, and the
  noindex-blocked check (D-24)

updates/202610270008_create_golem15_seo_robots.go

@@ -0,0 +1,33 @@
+package updates
+
+import (
+	"github.com/go-gormigrate/gormigrate/v2"
+	"gorm.io/gorm"
+)
+
+// createRobots creates golem15_seo_robots, the robots.txt singleton (D-22,
+// D-25), with its row 1: no rules and every AI bot allowed.
+var createRobots = []*gormigrate.Migration{
+	{
+		ID: "202610270008_create_golem15_seo_robots",
+		Migrate: func(tx *gorm.DB) error {
+			if err := tx.Exec(`CREATE TABLE golem15_seo_robots (
+	id BIGSERIAL PRIMARY KEY,
+	content TEXT NOT NULL DEFAULT '',
+	bots JSONB NOT NULL DEFAULT '{}',
+	created_at TIMESTAMP(0) NULL,
+	updated_at TIMESTAMP(0) NULL
+)`).Error; err != nil {
+				return err
+			}
+			return tx.Exec(`INSERT INTO golem15_seo_robots (id, content, bots, created_at, updated_at) VALUES (1, '', '{}', NOW(), NOW())`).Error
+		},
+		Rollback: func(tx *gorm.DB) error {
+			return tx.Exec(`DROP TABLE IF EXISTS golem15_seo_robots`).Error
+		},
+	},
+}
+
+func init() {
+	Register(createRobots...)
+}