Wires the previously-dead appendSessionRequest() DB function to a new
POST /api/tools/traffic-inspector/sessions/[id]/requests route.
appendSessionRequest now returns the inserted seq so callers can confirm order.
InspectorSessionRequestAppendSchema (1 MB cap) guards the endpoint.
Integration test covers seq increments 1-2-3, requestCount sync, 400/404 paths,
and stack-trace-free error responses.