docs: Add comprehensive live migration strategy guide

Commit notes

- Create LIVE_MIGRATION_STRATEGY.md with detailed production migration guidance - Add PRE_DEPLOY migration job to .do/app.yaml for automatic migrations - Update DEPLOYMENT.md with improved migration documentation - Update AGENTS.md with new db:check and db:mark-applied commands - Fix missing 0015_mollie_connect entry in migration journal

This guide covers: - Zero-downtime migration strategies - Breaking vs non-breaking changes - 3-phase deploy patterns for column changes - Rollback procedures - First-time deployment vs db:push transition

Co-authored-by: armin.naimi <[email redacted]>