fix(media): align Veo generation contract

[iamjr15]

Why

Production video generation sent two incompatible Veo options: an explicit audio toggle even though Veo 3.1 audio is native, and forced 1080p for four- and six-second clips even though 1080p requires eight seconds. Successful videos were also presented as generic files despite the product contract promising an in-chat preview.

What changed

• align text, reference-image, and extension requests with their mode-specific Veo 3.1 configuration
• reject invalid non-eight-second reference and extension requests before provider billing
• keep short text generations on the provider-default 720p path for lower latency
• lazy-load generated video previews from signed output URLs and add direct Files access
• share viewport preview state between image and video deliverables
• retain the shared double-shell Deliverables treatment merged in fix(web): unify deliverables double shell #275

Architecture and migration effects

No database migration or new environment variable. The Google AI BYOK boundary and R2 artifact ownership are unchanged.

Verification

• pnpm lint
• pnpm typecheck
• pnpm turbo build --force
• pnpm deadcode (existing Knip ignore hint only)
• pnpm architecture:check
• pnpm turbo skills:build
• schema probe: 4-second text accepted, 8-second extension accepted, 4-second extension rejected
• production acceptance test will run after the exact merged SHA is deployed to Cloudflare and Vercel