Add unprepare-db command to remove monitoring setup

Summary

Implement a CLI command that is the inverse of prepare-db - allowing users to cleanly remove all postgres_ai monitoring infrastructure from a database.

Motivation

Users who have set up monitoring with prepare-db need a way to cleanly remove it when:

• Decommissioning a database from monitoring
• Cleaning up test/dev environments
• Resetting monitoring configuration

Requirements

• Drop helper functions (explain_generic, table_describe)
• Drop postgres_ai.pg_statistic view
• Drop postgres_ai schema
• Revoke permissions (pg_monitor, CONNECT, SELECT on pg_index)
• Optionally drop the monitoring role (with --keep-role to preserve)
• Support Supabase provider (skip role operations)
• Confirmation prompt with --force to skip
• --print-sql for offline SQL plan review
• JSON output mode for automation

Usage

# Remove monitoring setup completely
postgresai unprepare-db postgresql://admin@host:5432/dbname

# Keep the role, only remove objects/permissions
postgresai unprepare-db <conn> --keep-role

# Preview SQL plan
postgresai unprepare-db <conn> --print-sql