Install the [mcp] extra in the production Docker image #33

Summary

Dockerfile.prod installs .[livekit] only. Production validation errors if MCP_SERVERS_CONFIG is set without the mcp package. Known-limitations tells operators to pip-install by hand inside the image.

Context

• Dockerfile.prod (pip install -e ".[livekit]")
• src/voxforge/infrastructure/security/production.py (import mcp check)
• docs/release/known-limitations.md
• Related docs issue Document how to enable MCP tools in the production image #16 (FAQ). This issue is the image fix.

Acceptance criteria

• Prod image includes mcp (or a documented build-arg defaulting on)
• MCP_SERVERS_CONFIG no longer fails the import check in a stock prod build
• Known-limitations MCP note updated
• docker-build CI still green

Do not pull MCP servers at runtime in CI.