feat(content-studio): add provider-agnostic video provider abstraction

[thxforall]

Parent: #508

Context

Phase 1A/1B job-backed scene clip generation landed in commit 1318f072, but the current pipeline is still xAI/job-slice oriented. This follow-up extracts the provider contract so Content Studio can support multiple video providers and all Grok Imagine generation modes without coupling UI/API code directly to xAI details.

Scope

• Define provider-agnostic video interfaces under the Content Studio video module.
• Implement Grok Imagine support for all 5 modes: text-to-video, image-to-video, reference-to-video, video editing, and video extension.
• Keep async job start/status/download behavior behind the provider abstraction.
• Preserve current pending/running semantics from the landed job pipeline.
• Add a manual/live smoke path for xAI end-to-end verification, gated so it is not run by default because it incurs provider cost.

Acceptance Criteria

• Existing job create/get/poll/retry routes can call the provider abstraction rather than xAI-specific helpers directly.
• Unit/focused tests cover provider selection and mode-specific request mapping.
• Live xAI smoke instructions are documented and opt-in only.

Out of Scope

• ffmpeg final composition.
• governance and cost-tracking integration.
• UI redesign beyond what is needed to select/trigger provider-backed generation.