Add database query step (MySQL, PostgreSQL, MongoDB)

[g3Bg2]

Description:
Execute database queries and store results in variables.

Tasks:

• Design DatabaseQuery step type
• Add connection configuration
• Support SQL queries
• Support MongoDB queries
• Handle query results
• Add connection pooling
• Test with all database types

Acceptance Criteria:

• Queries execute correctly
• Results stored in variables
• Connection management sound
• All databases supported