[C2S] Add Server-Sent Events (SSE) for real-time collection streaming

[pfefferle]

What

Implement the SWICG ActivityPub API SSE spec so C2S clients can subscribe to live updates on outbox and inbox collections instead of polling.

Key changes

• SSE stream endpoint (GET /actors/{id}/{collection}/stream) with push OAuth scope requirement
• Transient-based signal system for efficient polling — avoids unnecessary DB queries when no new items exist
• access_token query parameter fallback in OAuth Server for EventSource clients (browser EventSource API cannot send Authorization headers)
• eventStream property added to outbox and inbox collection responses
• proxyEventStream added to actor endpoints (proxy returns 501 for now — wp_remote_get doesn't support streaming)
• Event type mapping per spec: Create/Announce/Like → Add, Update → Update, Delete/Undo → Remove

Why

Enables C2S clients to receive real-time notifications of collection changes without polling, completing the push notification story for the C2S API.

Links

• PR: [C2S] Add Server-Sent Events (SSE) for real-time collection streaming #2945
• Spec: https://swicg.github.io/activitypub-api/sse