feat(core-backend): add package

.github/CODEOWNERS

@@ -92,6 +92,7 @@
 /packages/permission-log-controller    @MetaMask/wallet-integrations @MetaMask/core-platform
 /packages/remote-feature-flag-controller @MetaMask/extension-platform @MetaMask/mobile-platform
 /packages/foundryup                    @MetaMask/mobile-platform @MetaMask/extension-platform
+/packages/core-backend                 @MetaMask/core-platform @MetaMask/metamask-assets
 
 ## Package Release related
 /packages/account-tree-controller/package.json  @MetaMask/accounts-engineers @MetaMask/core-platform
@@ -170,3 +171,5 @@
 /packages/network-enablement-controller/CHANGELOG.md @MetaMask/metamask-assets @MetaMask/core-platform
 /packages/subscription-controller/package.json @MetaMask/web3auth @MetaMask/core-platform
 /packages/subscription-controller/CHANGELOG.md @MetaMask/web3auth @MetaMask/core-platform
+/packages/core-backend/package.json           @MetaMask/core-platform @MetaMask/metamask-assets
+/packages/core-backend/CHANGELOG.md           @MetaMask/core-platform @MetaMask/metamask-assets

packages/core-backend/CHANGELOG.md

@@ -0,0 +1,26 @@
+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [Unreleased]
+
+### Added
+
+- **Initial release of `@metamask/core-backend` package** - Core backend services for MetaMask serving as the data layer between Backend services and Frontend applications ([#6722](https://github.com/MetaMask/core/pull/6722))
+- **BackendWebSocketService** - WebSocket client providing authenticated real-time data delivery with:
+  - Connection management and automatic reconnection with exponential backoff
+  - Message routing and subscription management
+  - Authentication integration with `AuthenticationController`
+  - Type-safe messenger-based API for controller integration
+- **AccountActivityService** - High-level service for monitoring account activity with:
+  - Real-time account activity monitoring via WebSocket subscriptions
+  - Balance update notifications for integration with `TokenBalancesController`
+  - Chain status change notifications for dynamic polling coordination
+  - Account subscription management with automatic cleanup
+- **Type definitions** - Comprehensive TypeScript types for transactions, balances, WebSocket messages, and service configurations
+- **Logging infrastructure** - Structured logging with module-specific loggers for debugging and monitoring
+
+[Unreleased]: https://github.com/MetaMask/core/

packages/core-backend/LICENSE

@@ -0,0 +1,20 @@
+MIT License
+
+Copyright (c) 2025 MetaMask
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE