- Creates backend/ai/provider_config.py as a pure data module (no provider imports)
- ProviderConfig(BaseModel) with extra=forbid: provider_id, api_key, base_url, model, context_chars
- PROVIDER_DEFAULTS dict with all 10 providers and verbatim base_url/model/context_chars from RESEARCH.md
- SUPPORTS_JSON_MODE dict with gemini=False per JSON Compatibility Matrix (D-02)