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

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

backend/internal/service/watch_test.go

@@ -0,0 +1,50 @@
+package service
+
+import (
+	"testing"
+	"time"
+
+	"pargar/backend/internal/models"
+)
+
+func TestHeartbeatCapsDelta(t *testing.T) {
+	lesson := &models.Lesson{ID: 1, DurationSeconds: 100, CompletionThresholdPct: 85}
+	delta := 999.0
+	if delta > maxHeartbeatDelta {
+		delta = maxHeartbeatDelta
+	}
+	if delta > float64(lesson.DurationSeconds) {
+		delta = float64(lesson.DurationSeconds)
+	}
+	if delta != maxHeartbeatDelta {
+		t.Fatalf("expected cap %v, got %v", maxHeartbeatDelta, delta)
+	}
+}
+
+func TestHeartbeatCapsDeltaByWallClock(t *testing.T) {
+	last := time.Now().Add(-2 * time.Second)
+	elapsed := time.Since(last).Seconds()
+	wallCap := elapsed + wallClockSlack
+	delta := 15.0
+	if delta > wallCap {
+		delta = wallCap
+	}
+	if delta > maxHeartbeatDelta {
+		delta = maxHeartbeatDelta
+	}
+	if delta > 5 {
+		t.Fatalf("wall-clock cap should keep delta near ~3.5s, got %v", delta)
+	}
+	if delta < 2 {
+		t.Fatalf("wall-clock cap too aggressive: %v", delta)
+	}
+}
+
+func TestMaxHeartbeatDeltaConstant(t *testing.T) {
+	if maxHeartbeatDelta != 15.0 {
+		t.Fatalf("maxHeartbeatDelta changed unexpectedly: %v", maxHeartbeatDelta)
+	}
+	if wallClockSlack < 1 {
+		t.Fatalf("wallClockSlack too small: %v", wallClockSlack)
+	}
+}