fix(producer): honor variables + outputResolution in HTTP render server (#1152)

* fix(producer): honor variables + outputResolution in HTTP render server

The producer HTTP server's parseRenderOptions read only
fps/quality/workers/gpu/debug/entryFile/format from the request body.
`variables` and `outputResolution` were silently dropped, so any caller
of the server render path (the cloud-render sidecar that
experiment-framework POSTs to) got the composition's declared variable
defaults and its intrinsic dimensions regardless of what was requested.

RenderConfig already supports both fields (the local CLI `render`
command passes them); the server just never forwarded them. Wire them
through RenderInput, parseRenderOptions, and a shared buildRenderJobConfig
used by the sync + streaming handlers. outputResolution now drives the
same resolveDeviceScaleFactor supersampling path the local CLI uses, so a
4k render against a matching-aspect composition produces true 4k.

Validation: a non-object `variables` or an unknown `outputResolution`
returns a clean 400 instead of being silently ignored. Also extracts
resolvePreparedRenderOutput + parseRenderOverrides helpers to keep both
handlers DRY.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* fix(producer): reject non-string + alpha-incompatible outputResolution

Addresses review on #1152.

- A non-string `outputResolution` (e.g. a JSON number) was coerced to
  `undefined` by parseRenderOverrides and silently ignored — the same
  silent-drop this validation exists to prevent. Now rejected with a 400.
- `outputResolution` + an alpha format (webm/mov) is rejected up front:
  supersampling runs through a deviceScaleFactor the alpha capture path
  can't apply, so resolveDeviceScaleFactor throws mid-render. Guarding it
  here makes the producer self-defending for every caller (not just the
  CLI / external API), and closes the 1080p-webm regression window during
  the producer-honors-outputResolution rollout.

Extracted validateOutputResolutionOverride to keep validateRenderOverrides
under the complexity gate. +2 prepareRenderBody tests.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>