feat: objects table with M2M words/pictures/pairs

- objects: status enum, JSONB selections, notes, blocked_topic, auto-timestamps
- pairs placeholder table for future use
- Junction tables: object_words, object_pictures, object_pairs
- Full CRUD + link/unlink endpoints for all three relations
- README updated

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

src/index.js

@@ -27,6 +27,7 @@ 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'));
+app.use('/api/objects', auth, require('./routes/objects'));
 
 // 404
 app.use((req, res) => {