feat: categories table with full CRUD

- Fills out placeholder categories table with all fields
- Trilingual titles, status enum, difficulty level, auto-timestamps
- ALTER TABLE IF NOT EXISTS for safe migration on existing table
- /api/categories CRUD route, word_ids included in responses

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/index.js

@@ -26,6 +26,7 @@ app.get('/health', async (req, res) => {
 app.use('/api', auth, require('./routes/index'));
 app.use('/api/pictures', auth, require('./routes/pictures'));
 app.use('/api/words', auth, require('./routes/words'));
+app.use('/api/categories', auth, require('./routes/categories'));
 
 // 404
 app.use((req, res) => {