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.

backend/migrations/legacy/023_ensure_postgres_compatibility.js

@@ -0,0 +1,22 @@
+/**
+ * Ensure PostgreSQL compatibility for all insert operations
+ * This migration doesn't change the schema but ensures all tables
+ * are compatible with .returning() syntax
+ */
+
+exports.up = async function(knex) {
+  // This migration is informational only
+  // All insert operations should use .returning('id') going forward
+  
+  console.log('PostgreSQL compatibility check:');
+  console.log('- All INSERT operations should use .returning("id")');
+  console.log('- All date operations should use ISO strings');
+  console.log('- Boolean values are handled automatically by Knex');
+  
+  return Promise.resolve();
+};
+
+exports.down = async function(knex) {
+  // No rollback needed
+  return Promise.resolve();
+};