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

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

backend/internal/service/notify.go

@@ -0,0 +1,61 @@
+package service
+
+import (
+	"context"
+
+	"pargar/backend/internal/models"
+	"pargar/backend/internal/store"
+)
+
+// Pusher delivers realtime messages to a user's connected websockets.
+type Pusher interface {
+	Push(userID int64, msg any)
+}
+
+// Notifier persists in-app notifications and pushes them live.
+type Notifier struct {
+	store  *store.Store
+	pusher Pusher
+}
+
+func NewNotifier(st *store.Store, pusher Pusher) *Notifier {
+	return &Notifier{store: st, pusher: pusher}
+}
+
+func (n *Notifier) Notify(ctx context.Context, userID int64, category, typ, title, body, route string, data any) error {
+	enabled, err := n.store.PrefEnabled(ctx, userID, category)
+	if err != nil || !enabled {
+		return nil
+	}
+	notif := &models.Notification{
+		UserID:   userID,
+		Category: category,
+		Type:     typ,
+		Title:    title,
+		Body:     body,
+		Route:    route,
+	}
+	if data != nil {
+		notif.Data = mustJSON(data)
+	}
+	if err := n.store.CreateNotification(ctx, notif); err != nil {
+		return err
+	}
+	if n.pusher != nil {
+		n.pusher.Push(userID, map[string]any{
+			"type":   "notification",
+			"unread": true,
+			"item":   notif,
+		})
+	}
+	return nil
+}
+
+func (n *Notifier) NotifyMany(ctx context.Context, userIDs []int64, category, typ, title, body, route string, data any) error {
+	for _, uid := range userIDs {
+		if err := n.Notify(ctx, uid, category, typ, title, body, route, data); err != nil {
+			return err
+		}
+	}
+	return nil
+}