Real-time expansion: self-hostable broadcasting drivers and SSE

[eaguad1337]

Problem

Broadcasting depends solely on a proprietary hosted service. There is no self-hostable transport and no Server-Sent Events option, which limits real-time use without an external account.

Proposal

Expand real-time transport:

• A driver interface plus a Redis pub/sub driver and an open, self-hostable WebSocket server driver (Pusher-protocol-compatible target).
• A Server-Sent Events (SSE) response type for one-way streaming over plain HTTP.
• Keep the current hosted driver as one option among several.

Scope

Broadcasting driver interface + Redis driver + open WebSocket driver, SSE response type.

Definition of Done

• Implementation building on the existing broadcasting layer and response streaming.
• pytest covers driver selection, payload formatting, and SSE responses (suite green).
• Docs page + nav entry.
• Release notes entry.