feat(sessions): TTL enforcement and crash-safe cleanup

[Danelegend]

Sessions now carry an explicit TTL and are guaranteed to be torn down at or before that deadline even if the API service crashes.

• CreateSessionRequest gains ttl_seconds (default 900s, max 24h); CreateSessionResponse adds the absolute expires_at timestamp.
• Docker session containers are launched with --rm and sleep <ttl> so the container self-destructs at the deadline regardless of whether the API service is alive. The TTL also goes onto the container as the code-interpreter.expires-at label for the reaper.
• Kubernetes session pods set activeDeadlineSeconds=ttl so kubelet stops the executor container at the deadline; the deadline timestamp is also stored as a pod annotation so the reaper knows when to delete.
• A new background reaper task (in main.py's lifespan) runs once at startup — handling crash recovery for any sessions whose TTL elapsed while the service was down — and then every 30s thereafter.
• Both backends implement reap_expired_sessions (label-filtered list followed by per-session deletion) so the reaper is backend-agnostic.

Tests cover TTL bounds at the route layer, the active_deadline / expires-at metadata on each backend, and reaper behavior under happy path, missing annotation, invalid annotation, list failure, and partial delete-failure scenarios.