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/025_fix_email_queue_updated_at.js

@@ -0,0 +1,33 @@
+/**
+ * Fix email_queue table by ensuring it doesn't have updated_at column
+ * This migration addresses the PostgreSQL error where queries are trying to update
+ * a non-existent updated_at column
+ */
+
+exports.up = async function(knex) {
+  // First, check if the column exists
+  const hasUpdatedAt = await knex.schema.hasColumn('email_queue', 'updated_at');
+  
+  if (hasUpdatedAt) {
+    console.log('Found updated_at column in email_queue table, removing it...');
+    await knex.schema.table('email_queue', (table) => {
+      table.dropColumn('updated_at');
+    });
+  }
+  
+  // Also ensure the table has all required columns
+  const hasCreatedAt = await knex.schema.hasColumn('email_queue', 'created_at');
+  if (!hasCreatedAt) {
+    console.log('Adding missing created_at column to email_queue table...');
+    await knex.schema.table('email_queue', (table) => {
+      table.datetime('created_at').defaultTo(knex.fn.now());
+    });
+  }
+  
+  console.log('email_queue table schema fixed');
+};
+
+exports.down = async function(knex) {
+  // In the down migration, we don't add back updated_at since it shouldn't exist
+  // This is intentionally left minimal
+};