Add Android push notifications via Firebase Cloud Messaging

[johnnyluo]

Summary

• Implements processAndroidNotification() using the official Firebase Admin Go SDK (firebase.google.com/go/v4)
• The *messaging.Client is initialised once at worker startup from a service account credentials file and reused for every notification (no per-request re-initialisation)
• If firebase-credentials is empty/unset the worker starts normally and android notifications are skipped with a warning log (graceful degradation)

Changes

File	Change
service/notification.go	Add firebaseMessaging field, update constructor, implement processAndroidNotification()
config/config.go	Add FirebaseCredentials mapped to firebase-credentials config key
cmd/worker/main.go	Pass cfg.FirebaseCredentials to service constructor
go.mod / go.sum	Add firebase.google.com/go/v4 dependency

Notification payload

Mirrors the existing APNs notification:

• Title: "Vultisig Keysign request"
• Body: "Vault: <vault_name>"
• Data: {"message": "<qr_code_data>"}

Stale tokens (IsUnregistered FCM error) trigger automatic device removal from the database, matching the existing APNs behaviour.

Test plan

• Place a Firebase service account JSON at cmd/worker/firebase-service-account.json and set "firebase-credentials" in config.json
• Register an Android device via POST /register with device_type: "android"
• Trigger POST /notify with the matching vault_id and confirm the device receives the notification
• Verify stale-token cleanup: use an invalid FCM token, trigger a notification, confirm the device row is removed from the DB
• Run go test ./...

🤖 Generated with Claude Code