Push Notification Service

[iuricmp]

Implement a Push Notification Service that registers a user's device upon key creation or key recovery, and monitors blockchain activity for the associated address to notify the user of any incoming transactions.

Once a user creates or restores a key on the gnokey-mobile, we should:

• Register the device installation ID (Expo Push Token or similar)
• Register the associated blockchain address
• Listen for on-chain transactions related to that address
• Trigger a push notification to the app when relevant activity occurs

Acceptance Criteria:

• Register the user’s device installation ID and blockchain address on the notification backend
• Store the mapping of installationId <-> blockchain address securely
• Blockchain listener service monitors transactions for registered addresses
• Push notification sent to mobile device on new transactions
• Deduplicate notifications (avoid sending multiple for same TX)
• Secure the API with some auth/token mechanism

Optional Ideas

• Support per-chain monitoring (if gnokey-mobile will support multiple chains in future)
• Remove old entries from database
• Secure Database
• GDPR compliant