Introduce a configurable AI provider system (lib/ai-provider.ts) that
supports Gemini (default), MiniMax, and Siray.ai with automatic
fallback. The AI_PROVIDER env var selects the primary provider, and
on failure the system falls back to a secondary provider automatically.
- MiniMax M2.7 via OpenAI-compatible API (api.minimax.io/v1)
- Provider abstraction replaces hardcoded Gemini + Siray fallback
- 24 unit tests + 3 integration tests (vitest)
octopus