feat(auth): migrate Application model from MongoDB to MySQL

- Add Sequelize migration creating the 'application' table
- Add Application Sequelize model with slugify, toJSONFor, toAuthJSON
- Rewrite passport-headerapikey strategy to use DB.Application
- Rewrite applications.routes.js to use Sequelize (findAll, count,
  build/save, destroy) instead of Mongoose
- Fix pre-existing bug: passport was querying { encryptedKey } but the
  stored field is named 'apikey'
- Add Application to MySQL models index and relationships
- Remove Application from Mongoose models index and delete the schema

src/database/migrations/20260426000000-add-application.js

@@ -0,0 +1,63 @@
+'use strict';
+
+module.exports = {
+    async up(queryInterface, Sequelize) {
+        const transaction = await queryInterface.sequelize.transaction();
+        try {
+            await queryInterface.createTable('application', {
+                id: {
+                    type: Sequelize.STRING(36),
+                    allowNull: false,
+                    primaryKey: true
+                },
+                apikey: {
+                    type: Sequelize.STRING(255),
+                    allowNull: false
+                },
+                maskedkey: {
+                    type: Sequelize.STRING(20),
+                    allowNull: true
+                },
+                slug: {
+                    type: Sequelize.STRING(255),
+                    allowNull: false,
+                    unique: true
+                },
+                title: {
+                    type: Sequelize.STRING(255),
+                    allowNull: false
+                },
+                description: {
+                    type: Sequelize.TEXT,
+                    allowNull: true
+                },
+                authorId: {
+                    type: Sequelize.STRING(36),
+                    allowNull: true,
+                    references: {
+                        model: 'user',
+                        key: 'id'
+                    },
+                    onDelete: 'SET NULL'
+                },
+                createdAt: {
+                    type: Sequelize.DATE,
+                    allowNull: false
+                },
+                updatedAt: {
+                    type: Sequelize.DATE,
+                    allowNull: false
+                }
+            }, { transaction });
+
+            await transaction.commit();
+        } catch (err) {
+            await transaction.rollback();
+            throw err;
+        }
+    },
+
+    async down(queryInterface) {
+        await queryInterface.dropTable('application');
+    }
+};