feat: add /auth/feed endpoint for hydrated learning pairs

- GET /auth/feed?lang=sv&limit=20 (JWT, end-user allowed)
- Resolves {{uuid}} placeholders to word labels in all languages
- Includes picture URLs, pos/neg words per statement
- Fix migration seed: use full unique index (non-partial) for ON CONFLICT

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

src/index.js

@@ -24,6 +24,7 @@ app.get('/health', async (req, res) => {
 
 // Public routes
 app.use('/auth', require('./routes/auth'));
+app.use('/auth/feed', require('./routes/feed').router);
 
 // Routes — protected by Bearer token
 app.use('/api', auth, require('./routes/index'));