feat: add automatic database migration system

[productdevbook]

Summary

• Adds serverAssets config to bundle migrations in production build
• Adds shared migration utility with Drizzle __drizzle_migrations tracking
• Adds Nitro plugin for auto-migrate on startup (AUTO_MIGRATE env var)
• Adds API endpoint for manual migration trigger (GET /api/cron/migrate)

Usage

Auto-migrate (default)

# Migrations run automatically on server startup
AUTO_MIGRATE=true  # or just don't set it

Manual migrate

# Disable auto-migrate
AUTO_MIGRATE=false

# Trigger manually via API
curl http://localhost:3000/api/cron/migrate

# Or with CRON_SECRET for security
curl -H "Authorization: Bearer $CRON_SECRET" http://localhost:3000/api/cron/migrate

Why this approach?

Replaces #19's manual IF NOT EXISTS approach with proper Drizzle migration tracking:

• Drizzle tracks applied migrations in __drizzle_migrations table
• No need to modify SQL migration files
• Works in Docker production (migrations bundled via serverAssets)

Closes #19

Test plan

• Build includes migrations in .output/server/chunks/raw/
• Auto-migrate applies migrations on startup
• Manual migrate API works with AUTO_MIGRATE=false
• Migration tracking table records applied migrations
• New migrations are detected and applied

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