feat: add --compute flag for gpu encoding

[a-essawy]

What

Adds a --compute <auto|cpu|gpu> flag to rb ffmpeg, exposing the now-live GPU/compute routing option of the FFmpeg job type.

• auto (default behavior when omitted) — server auto-routes NVENC/CUDA commands to GPU
• gpu — forces GPU (NVENC on NVIDIA L4, Pro plan)
• cpu — forces CPU

Changes

• Parse --compute in the global-flag loop; it takes a value (added to the globalFlagsWithValue set so it's stripped from the FFmpeg args passed through to the executor).
• Validate the value against auto|cpu|gpu; an invalid value errors with a clear message and exits 2, matching the existing flag-error convention.
• Include compute in the submit params only when the user passed it, so default submissions are byte-identical to before.
• Help line: --compute <mode> Run on cpu or gpu hardware (auto, cpu, gpu; gpu needs Pro).

Tests

Four new cases in ffmpeg-command.test.ts: --compute gpu puts compute: "gpu" in params; auto/cpu accepted; invalid values rejected; omitting the flag leaves compute out of params.

Verification

• pnpm typecheck — clean
• pnpm test — 134 pass / 0 fail