Add user-managed environment variables and Metabase MCP server support

[devin-ai-integration]

Add user-managed environment variables and Metabase MCP server support

Summary

Implements a generic system for users to provide custom environment variables to MCP instances via the UI, then adds a new "Metabase" instance type that integrates the metabase-mcp-server and requires METABASE_URL and METABASE_API_KEY credentials.

Key Changes:

• New mcp_instance_environment_variables table with proper entity relationships
• UI for managing key-value pairs on instance detail pages
• Environment variables passed to Docker containers during creation
• New Metabase MCP server Docker image and configuration
• Comprehensive unit test coverage (15 tests across 4 test classes)

Review & Testing Checklist for Human

This is a medium-risk change that touches database, container orchestration, and user input handling.

• Test environment variable UI end-to-end - Create an instance, add/modify/delete environment variables, verify they persist and appear in container
• Verify container environment merging - Check that both instance type defaults AND user-defined variables are passed to containers correctly
• Test Metabase instance creation - Create a Metabase instance with required METABASE_URL and METABASE_API_KEY, verify container starts successfully
• Security review - Environment variables are stored as plain text in database - confirm this is acceptable for the intended use case
• Validate key format restrictions - Test that invalid environment variable names (spaces, special chars) are properly rejected

Recommended Test Plan:

1. Run migrations on staging environment
2. Create test Metabase instance with real credentials
3. Add various environment variables (valid/invalid keys, special characters in values)
4. Verify container logs show correct environment variables
5. Test UI edge cases (empty keys, mismatched arrays, very long values)

Notes

• Database Migration Note: Two migrations were required due to SQL reserved word issue with key column (renamed to env_key)
• Container Integration: Fixed existing gap where instance type environment variables weren't being merged into containers
• Testing: All 15 unit tests pass, covering entity behavior, domain logic, controller validation, and container integration

Requested by: @manuelkiessling
Link to Devin run: https://app.devin.ai/sessions/a0eaccf3cd904767b484d892cdbf9aeb

[devin-ai-integration]

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