feat: words table, M2M with pictures and categories

- words table with trilingual titles, status enum, difficulty level, timestamps
- word_pictures junction table (M2M words <-> pictures)
- categories placeholder table
- word_categories junction table (M2M words <-> categories)
- Auto-timestamps on status change (requested/published/blocked)
- Full CRUD + link/unlink endpoints for pictures and categories
- README updated with schema and endpoints

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

src/index.js

@@ -25,6 +25,7 @@ app.get('/health', async (req, res) => {
 // Routes — protected by Bearer token
 app.use('/api', auth, require('./routes/index'));
 app.use('/api/pictures', auth, require('./routes/pictures'));
+app.use('/api/words', auth, require('./routes/words'));
 
 // 404
 app.use((req, res) => {