test: integration — full session lifecycle with CreateIngress/DeleteIngress for camera role

[courtnaejames1]

Summary

Adds an end-to-end integration test covering the complete production session lifecycle through the signaling server's gRPC API.

What was added

A new TestLifecycle_FullSessionFlow test in backend/internal/signaling/grpc_integration_test.go that exercises the full flow a real production would follow:

1. CreateSession — director creates a session and receives a valid room code
2. JoinSession (director) — joins with role "director", asserts a JWT token and LiveKit URL are returned
3. JoinSession (camera) — camera join, asserting a WHIP URL and stream key returned, with CreateIngress called against LiveKit for each
4. CloseSession — director closes the session, triggering DeleteIngress cleanup for all stored ingress IDs
5. Post-close assertions — both director and camera JoinSession calls return FailedPrecondition, confirming the session cannot be rejoined

What this covers that unit tests don't

The existing unit tests in grpc_test.go verify ingress behaviour with mocks in isolation. This test runs against the real LiveKit and Ingress services in the dev stack, proving the full control plane flow works end-to-end — room code generation, Redis state, LiveKit ingress lifecycle, and session status transitions all in one pass.

How to run

cd backend
go test -race -count=1 -tags=integration -run TestLifecycle_FullSessionFlow ./internal/signaling/

Closes #140