Feature: Dynamic UI forms from model metadata

Problem

UI forms are static - users can't see available voices, languages, or valid parameter ranges for each model without guessing or looking up docs.

Solution

Providers already know this data. Expose it via a metadata endpoint (e.g., GET /v1/models/{id}/metadata) using JSON Schema so the UI can dynamically render:

• dropdowns for enums (voices, formats)
• sliders for ranges (speed, temperature)
• proper input types for each parameter

No more hardcoded forms.

My Previous attempt

PR #932 - closed because I wasn't familiar enough with the codebase to implement it properly. Open to revisiting once we agree on design.