Claude Code, KiloCode, audio uploads — compatibility round

• Anthropic-format 429s on /v1/messages — Claude Code now retries cleanly instead of erroring out.
• Multipart /v1/audio/transcriptions so tools that default to form-data upload without manual reshaping.
• Better error messages — useful 400s reach you intact instead of generic upstream noise.
• Large audio uploads return a proper 413 instead of timing out.

Fixes & QOL across the model picker, integration page, and stream-finalization edge cases.