Add Context APIs and refactor vocab models

Introduce Context and ContextGuide features: add Sequelize models (models/Context.js, models/ContextGuide.js), controllers (controllers/contextController.js, controllers/contextGuideController.js) and authenticated route handlers (routes/contextRoutes.js, routes/contextGuideRoutes.js). Wire the new routes into app.js and export the models from models/index.js. Refactor vocabulary: remove VocabForm, VocabMapping and VocabRelation models and relationships, update models/Vocab.js schema and indexes, and add migrate-vocab.js to drop/recreate the vocab table for the new schema. Also add a lesson editor UI (public/lesson-editor.html) and a small cleanup in models/Lesson.js.

routes/contextRoutes.js

@@ -0,0 +1,25 @@
+const express = require('express');
+const router = express.Router();
+const contextController = require('../controllers/contextController');
+const { authenticateToken } = require('../middleware/auth');
+
+/**
+ * Context Routes
+ */
+
+// Get all contexts
+router.get('/', authenticateToken, contextController.getAllContexts);
+
+// Get context by UUID
+router.get('/:id', authenticateToken, contextController.getContextById);
+
+// Create context
+router.post('/', authenticateToken, contextController.createContext);
+
+// Update context
+router.put('/:id', authenticateToken, contextController.updateContext);
+
+// Delete context
+router.delete('/:id', authenticateToken, contextController.deleteContext);
+
+module.exports = router;