feat: add KIE.AI video provider

[RomuloGatto]

Summary

• add a new kie_video provider for KIE.AI Market video generation
• support Seedance 2 / Seedance 2 Fast and Kling v3 Turbo text/image-to-video variants
• upload local image references through KIE's file upload API before generation
• handle both documented KIE upload responses (data.fileUrl) and live file-stream responses (data.downloadUrl)
• document KIE_API_KEY in .env.example and provider docs
• update the phase 3 TTS provider contract to include the already-discovered Doubao provider
• include the updated diagram.png

Verification

• python -m pytest tests/tools/test_kie_video.py -q → 6 passed
• python -m compileall tools/video/kie_video.py → ok
• Registry smoke test with KIE_API_KEY=dummy confirmed kie_video is discovered as a video_generation provider
• Live Caio image-to-video smoke test through KIE succeeded locally and produced a 720x1280, 4.04s MP4

[gioOrchid]

wow this is a must!