add shell rental

[luke-lombardi]

Summary by cubic

Adds marketplace rentals so buyers can reserve GPUs on a specific seller machine and launch machine‑pinned shells or pods. Includes rental APIs, scheduler pinning and wait logic, Redis-backed rentals with per-machine locking, usage emission for billing, and SDK/OpenAPI updates with pricing.

• New Features

  • Rental model and Redis storage with per-buyer/machine indexes and a per-machine lock; emits rental usage during reconcile with a timeout.
  • Gateway APIs: create/list/delete rentals and POST /marketplace/rentals/{rental_id}/launch for shells/pods; OpenAPI/proto generated.
  • Machine pinning for shells/pods via StubConfigV1.machine_id and types.ContainerRequest.machine_id; scheduler restricts to the pinned worker and waits if busy; new unit tests.
  • SDK updates: rental messages, machine_id, and price_per_gpu_hour_cents on listings/offers.

• Refactors

  • Moved empty stub object helper to pkg/abstractions/common; gateway uses it.
  • Added Redis key constants and wiring in gateway compute service (RedisClient, rentalUsage).
  • Proto/offer shaping tweaks and test expansions.

^{Written for commit a4729c0. Summary will update on new commits.}

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