feat(social-controllers): add realtime feed service

[xavier-brochard]

Summary

• Add SocialRealtimeService for subscribing to Social realtime feed events through the shared backend WebSocket connection.
• Align the @metamask/core-backend dependency with the v11 workspace package.
• Serialize connection and subscription setup to prevent duplicate subscriptions during synchronous CONNECTED notifications.
• Retry subscription setup when an overlapping activation or cleanup completes without a live subscription.
• Keep subscription ownership local to SocialRealtimeService so existing backend subscriptions are not adopted or unsubscribed accidentally.
• Export the service and event types from @metamask/social-controllers.

Testing

• yarn workspace @metamask/social-controllers run test --no-coverage
• yarn workspace @metamask/social-controllers run build
• yarn workspace @metamask/social-controllers run changelog:validate
• yarn workspace @metamask/social-controllers run lint:tsconfigs
• yarn lint:misc --check packages/social-controllers/src/SocialRealtimeService.ts packages/social-controllers/src/SocialRealtimeService.test.ts
• yarn lint:oxlint packages/social-controllers/src/SocialRealtimeService.ts packages/social-controllers/src/SocialRealtimeService.test.ts packages/social-controllers/src/index.ts packages/social-controllers/src/index.test.ts

---

Note

Medium Risk
New WebSocket subscription lifecycle and async edge cases (reconnect, race on deactivate) could affect feed freshness if mis-integrated, but failures are best-effort and scoped to Social realtime.

Overview
Adds SocialRealtimeService to @metamask/social-controllers so clients can receive live Social feed updates over the existing BackendWebSocketService connection (channel social.v1.feed.all), without opening a separate socket.

Consumers call setActive(true|false) to subscribe/unsubscribe (e.g. when the feed tab is focused), optionally gated by isEnabled. Valid feed-item payloads are validated and forwarded to listeners; reconnects trigger resubscription and optional addReconnectListener callbacks. Subscription setup is serialized to avoid duplicates during rapid connect/disconnect and overlapping activate/deactivate.

The package now depends on @metamask/core-backend ^11.0.0, exports the service and related types from the public index, and documents the dependency in the monorepo graph. This is a new exported capability—SocialController is not wired to it in this PR.

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