feat(backend): add domain store, learning services, and seed data

Persist LMS entities and implement hearts/watch rules plus curriculum bootstrap.

backend/internal/service/util.go

@@ -0,0 +1,21 @@
+package service
+
+import (
+	"encoding/json"
+	"fmt"
+	"time"
+)
+
+func mustJSON(v any) json.RawMessage {
+	b, err := json.Marshal(v)
+	if err != nil {
+		return json.RawMessage(`{}`)
+	}
+	return b
+}
+
+// ISOWeek returns the ISO-8601 week key like "2026-W37".
+func ISOWeek(t time.Time) string {
+	year, week := t.ISOWeek()
+	return fmt.Sprintf("%d-W%02d", year, week)
+}