Adds the ability to connect to OpenAI, Mistral, Google, Cohere, Together,
Fireworks, and Perplexity from the Studio chat interface.
- Provider configs stored in SQLite (no API keys persisted)
- RSA-2048 key pair generated at startup for client-side key encryption
- httpx proxy client streams SSE responses in OpenAI-compatible format
- New /api/providers routes: registry, CRUD, test, models
- /v1/chat/completions routes to external provider when provider fields present
- Integration test suite covering CRUD, connection, model listing, and inference
- Frontend spec doc with full API contract
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