feat: add unprepare-db command and pg_stat_statements to prepare-db

Closes #95 (closed)

Summary

New: unprepare-db command

• Implements the inverse of prepare-db, allowing users to cleanly remove all postgres_ai monitoring infrastructure from a database
• Drops helper functions (explain_generic, table_describe)
• Drops postgres_ai.pg_statistic view and postgres_ai schema
• Revokes permissions (pg_monitor, CONNECT, SELECT on pg_index)
• Optionally drops the monitoring role (--keep-role to preserve)

New: pg_stat_statements extension

• prepare-db now creates pg_stat_statements extension (with IF NOT EXISTS)
• unprepare-db does NOT drop it (may have been pre-existing)

Improved: Schema safety

• Removed IF NOT EXISTS from CREATE SCHEMA postgres_ai
• If schema already exists, prepare-db will fail with a clear error
• User should run unprepare-db first to clean up

Features

• Supports Supabase provider (skips role operations)
• Confirmation prompt with --force to skip
• --print-sql for offline SQL plan review
• JSON output mode for automation

Usage

# Prepare database for monitoring
postgresai prepare-db postgresql://admin@host:5432/dbname --password <mon-pass>

# 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

Test plan

• Unit tests for buildUninitPlan and applyUninitPlan
• Tests for special character handling in identifiers
• Tests for pg_stat_statements (created, not dropped)
• Tests for schema creation (no IF NOT EXISTS)
• CLI tests for unprepare-db command
• Manual testing against real PostgreSQL database