feat: add model prompt length limits

[nblog]

Summary

• Move Playground prompt length limits into model catalog params using params.prompt.maxLength.
• Make Playground prompt input and backend generation validation read the generated model catalog.
• Add prompt limits for Seedance, PixVerse, and HappyHorse while preserving the 2000-character default fallback.
• Change Seedance 2.0 T2V/I2V/R2V default resolution to 720p.

Test plan

• .venv/bin/python scripts/validate_model_catalog.py
• /usr/bin/pytest -q tests/test_playground_prompt_limits.py
• git diff --cached --check
• staged diff sensitive scan
• Frontend Vitest not rerun in this environment: WSL has no Linux Node, and Windows Node cannot load the Linux Rollup optional package from the existing node_modules.