🛠️ Add automated Heroku-24 migration script

[matrixise]

This PR adds an automated migration script to handle the Heroku-22 to Heroku-24 stack upgrade.

What's included

1. Migration Script (scripts/migrate-heroku-24.sh)

A comprehensive bash script that automates the entire migration process with built-in safety features:

Commands:

• ./scripts/migrate-heroku-24.sh status - Check current stack status
• ./scripts/migrate-heroku-24.sh migrate - Run migration to heroku-24
• ./scripts/migrate-heroku-24.sh rollback - Rollback to heroku-22

Features:

• ✅ Automatic database and config backups before migration
• ✅ Maintenance mode handling (enables/disables automatically)
• ✅ Automatic smoke tests after deployment (homepage, admin, logs)
• ✅ Built-in rollback capability with one command
• ✅ Confirmation prompts before critical actions
• ✅ Detailed logging to timestamped files
• ✅ Error handling with automatic rollback offer on failure

2. Complete Documentation (scripts/README-heroku-migration.md)

Full documentation including:

• Prerequisites and setup
• Usage examples
• Features and safety mechanisms
• Troubleshooting guide
• Post-migration monitoring instructions
• Environment variables
• Exit codes reference

Testing

The script has been tested successfully:

• ✅ status command works correctly
• ✅ Shows current stack: heroku-22
• ✅ PostgreSQL 17.6 info displayed
• ✅ Identifies readiness for migration

Migration readiness

All 5 phases of migration testing have been completed successfully (see docs/heroku-24-migration-tests.md):

• ✅ Phase 1-3: Verified production state, Python 3.13 compatibility, dependencies
• ✅ Phase 4-5: Staging deployment successful with production data

The migration is ready for production.

Closes #180