List Imported Frames Internal

Agent polls this every few seconds to discover Recall.ai-bridged screenshare frames written by the Phase A WebSocket sink (source='imported').

Returns the newest frames after since (a captured_at unix timestamp), so the caller advances since to the latest captured_at it has seen and never re-fetches old frames. Internal-secret auth — this is the same channel the agent already uses for /internal/trace + /internal/results.

Phase B of the Recall screen-share bridge: the agent uses the most recent frame returned here to populate its _latest_frame field so per-turn LLM injection picks up the screen content without needing a LiveKit RTC publisher on the backend.