Add k8s request fraction

[clementblaise]

Changes

• Fix Docker/K8s memory_mb semantics mismatch by making CPU/memory requests configurable fractions of task limits, allowing overcommit locally while keeping prod safety
• Add CPU/memory requests to Kaniko build job containers
• Fix orphaned eval pods surviving screener container restarts
• Move blocking K8s exec/tar upload/download I/O off the event loop onto threads
• Enlarge the default thread pool at startup to stop long-lived exec-stream threads from starving heartbeat/cancellation-check requests