Add provider abstraction for model-agnostic routing

[suboss87]

Summary

Add a provider abstraction layer so users can route to Runway, Kling, Veo, or any future video gen API without rewriting the pipeline.

Proposed approach

1. Define a VideoProvider protocol in app/services/providers/base.py
2. Move current ModelArk logic to app/services/providers/byteplus.py
3. Add provider selection via VIDEO_PROVIDER=byteplus env var
4. Each provider implements: create_task(), poll_status(), get_result()

Why this matters

• Removes vendor lock-in
• Positions SeedCamp as the routing layer, not an API wrapper
• Enables cost-optimized routing across providers