feat(loyalty): notify transfer recipients through FCM

The recipient of a transfer now gets a push through FCM instead of a
WhatsApp message (docs/prd-point-coin.md F5).

Customers had nowhere to keep FCM tokens: user_devices only holds staff
devices. Migration 000096 adds customer_devices, and the customer app
registers with PUT /customer/devices { device_id, fcm_token, platform,
app_version } after login and whenever FCM refreshes the token, and
unregisters with DELETE /customer/devices/:device_id on logout. A token
belongs to one customer only: registering it takes it away from whoever
had it on that phone before, so they stop getting this customer's
notifications.

The push goes to every device of the recipient after the commit, titled
"EnakPoint masuk" or "EnakCoin masuk", with type WALLET_TRANSFER_IN, the
TRANSFER_IN transaction id, the group id, the currency and the amount in
its data so the app can open it. A retried transfer sends nothing again. It
stays best effort: no device, FCM not configured or FCM failing is logged
and never undoes the transfer.

The app builds one FCM client and shares it between staff notifications
and customer pushes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/processor/customer_device_processor.go

@@ -0,0 +1,77 @@
+package processor
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"strings"
+	"time"
+
+	"github.com/google/uuid"
+
+	"apskel-pos-be/internal/logger"
+	"apskel-pos-be/internal/repository"
+)
+
+// ErrInvalidCustomerDevice wraps every rejection of a device registration.
+var ErrInvalidCustomerDevice = errors.New("invalid customer device")
+
+type customerPushSender interface {
+	SendMulticastNotification(ctx context.Context, tokens []string, title string, body string, data map[string]string) error
+}
+
+// CustomerDeviceProcessor keeps the customer app's FCM tokens and sends push
+// notifications to a customer's devices.
+type CustomerDeviceProcessor struct {
+	repo repository.CustomerDeviceRepository
+	// Nil when FCM is not configured; notifications are then skipped.
+	fcm customerPushSender
+	now func() time.Time
+}
+
+func NewCustomerDeviceProcessor(repo repository.CustomerDeviceRepository, fcm customerPushSender) *CustomerDeviceProcessor {
+	return &CustomerDeviceProcessor{repo: repo, fcm: fcm, now: time.Now}
+}
+
+// Register saves the FCM token the app got for this device. The app calls it after
+// login and whenever FCM gives it a new token.
+func (p *CustomerDeviceProcessor) Register(ctx context.Context, device repository.CustomerDevice) error {
+	device.DeviceID = strings.TrimSpace(device.DeviceID)
+	device.FCMToken = strings.TrimSpace(device.FCMToken)
+	switch {
+	case device.DeviceID == "" || len(device.DeviceID) > 255:
+		return fmt.Errorf("%w: device_id is required, at most 255 characters", ErrInvalidCustomerDevice)
+	case device.FCMToken == "" || len(device.FCMToken) > 512:
+		return fmt.Errorf("%w: fcm_token is required, at most 512 characters", ErrInvalidCustomerDevice)
+	}
+	if device.Platform != nil {
+		platform := strings.ToLower(strings.TrimSpace(*device.Platform))
+		if platform != "android" && platform != "ios" && platform != "web" {
+			return fmt.Errorf("%w: platform must be android, ios or web", ErrInvalidCustomerDevice)
+		}
+		device.Platform = &platform
+	}
+	return p.repo.Register(ctx, device, p.now())
+}
+
+// Unregister forgets a device, so it stops getting the customer's notifications.
+func (p *CustomerDeviceProcessor) Unregister(ctx context.Context, customerID uuid.UUID, deviceID string) error {
+	return p.repo.Unregister(ctx, customerID, strings.TrimSpace(deviceID))
+}
+
+// Notify pushes a notification to every device of the customer through FCM. A
+// customer without a registered device gets nothing, which is not an error.
+func (p *CustomerDeviceProcessor) Notify(ctx context.Context, customerID uuid.UUID, title, body string, data map[string]string) error {
+	if p.fcm == nil {
+		logger.NonContext.Info(fmt.Sprintf("FCM is not configured; not notifying customer %s", customerID))
+		return nil
+	}
+	tokens, err := p.repo.ListTokens(ctx, customerID)
+	if err != nil {
+		return err
+	}
+	if len(tokens) == 0 {
+		return nil
+	}
+	return p.fcm.SendMulticastNotification(ctx, tokens, title, body, data)
+}