feat(store): persist garmin_connected_at, set once on first successful auth

backend/internal/store/profile_test.go

@@ -107,3 +107,44 @@ func TestProfile_DefaultsThenUpdate(t *testing.T) {
 		t.Errorf("TargetBrightenPct after update = %v, want 50", got.TargetBrightenPct)
 	}
 }
+
+func TestMarkGarminConnected_SetsTimestampOnceOnFirstCall(t *testing.T) {
+	db := openTestDB(t)
+	ctx := context.Background()
+	userID, err := db.ProvisionUser(ctx, "test-sub", "Test")
+	if err != nil {
+		t.Fatalf("ProvisionUser: %v", err)
+	}
+
+	before, err := db.GetProfile(ctx, userID)
+	if err != nil {
+		t.Fatalf("GetProfile: %v", err)
+	}
+	if before.GarminConnectedAt != nil {
+		t.Fatalf("expected a fresh profile to have nil GarminConnectedAt, got %v", *before.GarminConnectedAt)
+	}
+
+	if err := db.MarkGarminConnected(ctx, userID); err != nil {
+		t.Fatalf("MarkGarminConnected: %v", err)
+	}
+	afterFirst, err := db.GetProfile(ctx, userID)
+	if err != nil {
+		t.Fatalf("GetProfile after first mark: %v", err)
+	}
+	if afterFirst.GarminConnectedAt == nil {
+		t.Fatal("expected GarminConnectedAt to be set after MarkGarminConnected")
+	}
+	firstValue := *afterFirst.GarminConnectedAt
+
+	// A second call must not change the recorded first-connection time.
+	if err := db.MarkGarminConnected(ctx, userID); err != nil {
+		t.Fatalf("MarkGarminConnected (second call): %v", err)
+	}
+	afterSecond, err := db.GetProfile(ctx, userID)
+	if err != nil {
+		t.Fatalf("GetProfile after second mark: %v", err)
+	}
+	if afterSecond.GarminConnectedAt == nil || *afterSecond.GarminConnectedAt != firstValue {
+		t.Fatalf("GarminConnectedAt changed on second call: first=%q second=%v", firstValue, afterSecond.GarminConnectedAt)
+	}
+}