Fix Realtime Fetch Signal Leak

Fix RSS memory leak in the realtime proxy routes. /realtime/v1/runs, /realtime/v1/runs/:id, and /realtime/v1/batches/:id called fetch() into Electric with no abort signal, so when a client disconnected mid long-poll, undici kept the upstream socket open and buffered response chunks that would never be consumed — retained only in RSS, invisible to V8 heap tooling. Thread getRequestAbortSignal() through RealtimeClient.streamRun/streamRuns/streamBatch to longPollingFetch and cancel the upstream body in the error path. Isolated reproducer showed ~44 KB retained per leaked request; signal propagation releases it cleanly.