Otel: thread request context through streaming chat path (#129 review)

The legacy SSE streaming path (chat_turn_stream -> run_streaming_turn ->
run_bootstrap/continuation_streaming) built its LLM spans from
Context::current(), which is always the empty root context here: the codebase
threads OTel contexts explicitly and never attaches one, there is no
context-aware tokio layer, and the turn runs on a tokio::spawn. So every
llm.request span on that path was an orphaned root span, uncorrelated with the
HTTP request.

Thread the handler's extracted request context down through
chat_turn_stream -> chat_turn_stream_inner -> run_streaming_turn ->
run_bootstrap_streaming / run_continuation_streaming, and use it for the
describe_image and agentic-loop calls. Mirrors the non-streaming path, which
already threads parent_cx -> loop_cx.

Also rename the block-scoped db 'cx' in run_streaming_turn to db_cx so it no
longer shadows the new parameter.

src/ai/handlers.rs

@@ -1667,10 +1667,12 @@ pub struct ChatSwitchBranchRequest {
 /// Returns `text/event-stream` with one event per chat stream event.
 #[post("/insights/chat/stream")]
 pub async fn chat_stream_handler(
+    http_request: HttpRequest,
     claims: Claims,
     request: web::Json<ChatTurnHttpRequest>,
     app_state: web::Data<AppState>,
 ) -> HttpResponse {
+    let parent_context = extract_context_from_request(&http_request);
     let mut request = request.0;
     // Stored owner defaults fill what the client omitted (request → pref →
     // hard default; the hard default still applies downstream).
@@ -1716,7 +1718,7 @@ pub async fn chat_stream_handler(
     };
 
     let service = app_state.insight_chat.clone();
-    let events = service.chat_turn_stream(chat_req);
+    let events = service.chat_turn_stream(chat_req, parent_context);
 
     // Map ChatStreamEvent → SSE frame bytes.
     let sse_stream = futures::stream::StreamExt::map(events, |ev| {