feat(ai): streaming chat endpoint with live tool events

Add LlmClient::chat_with_tools_stream and SSE endpoint
POST /insights/chat/stream that emits text deltas, tool_call /
tool_result pairs, truncated notice, and a terminal done frame as the
agentic loop runs.

- Ollama: parses NDJSON from /api/chat stream, accumulates content
  deltas, emits Done with tool_calls from the final chunk.
- OpenRouter: parses OpenAI-compatible SSE, reassembles tool_call
  argument deltas by index, asks for stream_options.include_usage.
- InsightChatService spawns the loop on a tokio task, feeds events
  through an mpsc channel, persists training_messages at the end.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/main.rs

@@ -1357,6 +1357,7 @@ fn main() -> std::io::Result<()> {
                 .service(ai::get_available_models_handler)
                 .service(ai::get_openrouter_models_handler)
                 .service(ai::chat_turn_handler)
+                .service(ai::chat_stream_handler)
                 .service(ai::chat_history_handler)
                 .service(ai::chat_rewind_handler)
                 .service(ai::rate_insight_handler)