Description

This PR implements the DeepSeek provider for the LLM Proxy and Chat, resolving issue #1846.

Note: This supersedes PR #2259 which was closed during a git history squash to ensure a clean commit log.

Changes

• Added deepseek to SupportedProviders.
• Added configuration for ARCHESTRA_DEEPSEEK_BASE_URL and API key.
• Implemented DeepSeekAdapter (using OpenAI compatibility layer).
• Added proxy routes for DeepSeek.
• Updated frontend API key form to include DeepSeek.

Checklist

• Provider registered in constants
• Config updated
• Adapter implemented
• Routes added
• Frontend updated

/claim #1846