Reorganize migrations for new vs existing deployments

- Created core/ directory for essential migrations that always run
- Created legacy/ directory for migrations only needed when upgrading
- New deployments will only run core migrations for a clean database
- Existing deployments will run all migrations in proper sequence
- Fixed duplicate migration numbers (014 and 027)
- Updated migration runners to handle new directory structure
- Added README explaining the migration organization

This change optimizes deployment for new users who will get a clean
schema without running unnecessary upgrade migrations.

.gitignore

@@ -63,6 +63,8 @@ test-archiver/
 backend/.swarm/
 .claudedocs/
 backend/data/
+backend/docs/
+backend/logs/
 logs/
 storage/
 data/