feat: streaming API responses

[rajnavakoti]

Description

Add streaming support to the /ask API endpoint and the Ask viewer page. Currently the full answer is generated before returning, causing long waits. Stream the retrieval stages and synthesis for better UX.

Acceptance Criteria

• /ask endpoint supports Accept: text/event-stream for SSE streaming
• Streams: retrieval progress → entity matches → synthesis tokens → gap detection
• Ask page in viewer shows streaming answer as it's generated
• Non-streaming mode still works for backward compatibility
• CLI cb ask shows streaming output by default

Out of Scope

• WebSocket support
• Streaming for extraction pipeline