Add direct MiniMax chat provider

[octo-patch]

Reason: Add a direct MiniMax chat provider so the Ask the Atlas RAG path can stream from MiniMax's Anthropic Messages-compatible endpoint instead of being OpenRouter-only.

Changes

• Add lib/chat-provider.js: a single transport-selection module that resolves the active chat provider (openrouter, default, or minimax) and builds the streaming chat request for each. For MiniMax it targets the Anthropic Messages-compatible endpoint, supports the global (api.minimax.io) and China (api.minimaxi.com) regional hosts via MINIMAX_REGION, and serves the current MiniMax text models (MiniMax-M3, MiniMax-M2.7) via the MINIMAX_MODELS list. The module also exposes transport-agnostic SSE accessors (chatStreamDelta, chatStreamModel, chatStreamUsage) covering both OpenRouter (choices[].delta.content / model / usage) and MiniMax Anthropic (delta.text / message.model / usage) chunk shapes.
• Wire api/chat.js to use the new module: provider config is resolved once at module load, the API-key gate and the streaming fetch are dispatched per provider, and the SSE loop reads deltas/model/usage through the shared accessors. The OpenRouter waterfall is unchanged in behavior; the MiniMax path is opt-in via CHAT_PROVIDER=minimax.
• Add tests/chat-provider.test.js covering region resolution, model-list override, and the request shape for both transports.
• Document the new CHAT_PROVIDER, MINIMAX_API_KEY, MINIMAX_REGION, and MINIMAX_MODELS environment variables in the README env table.

Checks

• node --check api/chat.js
• node --check lib/chat-provider.js
• node --test tests/chat-provider.test.js (10/10 passing)
• node --test tests/openrouter.test.js (existing OpenRouter tests still green)

MiniMax is reached through its OpenAI-compatible and Anthropic-compatible endpoints; the chatbot uses the Anthropic-compatible transport so the existing streaming parser keeps working without per-provider branching.

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