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/017_add_token_revocation_tables.js

@@ -0,0 +1,34 @@
+exports.up = function(knex) {
+  return knex.schema
+    // Table for individual token revocations
+    .createTable('revoked_tokens', table => {
+      table.increments('id').primary();
+      table.string('token_id').notNullable().unique(); // JWT ID or generated ID
+      table.integer('user_id').nullable(); // User who owned the token
+      table.string('token_type', 20); // admin, gallery, etc.
+      table.timestamp('revoked_at').defaultTo(knex.fn.now());
+      table.timestamp('expires_at').notNullable(); // When token would have expired
+      table.string('reason', 100); // password_change, logout, compromised, etc.
+      table.text('metadata'); // Additional JSON data
+      
+      // Indexes for performance
+      table.index('token_id');
+      table.index('user_id');
+      table.index('expires_at'); // For cleanup
+    })
+    // Table for user-level revocations (revoke all tokens before a certain time)
+    .createTable('user_token_revocations', table => {
+      table.integer('user_id').primary();
+      table.timestamp('revoked_at').notNullable();
+      table.string('reason', 100);
+      
+      // Index for quick lookups
+      table.index('revoked_at');
+    });
+};
+
+exports.down = function(knex) {
+  return knex.schema
+    .dropTableIfExists('user_token_revocations')
+    .dropTableIfExists('revoked_tokens');
+};