feat: Feed-Pagination – erledigte und vom Client gelieferte Pairs ausschließen

GET /auth/feed schließt jetzt Pairs aus user_pair_progress (cross-session)
sowie per ?exclude=<uuids> übergebene, bereits geladene Pairs (In-Session)
aus. Leere Antwort signalisiert dem Client: keine weiteren Karten.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/routes/feed.js

@@ -41,10 +41,18 @@ router.get('/', requireJwt, async (req, res, next) => {
   try {
     const lang   = ['de', 'en', 'sv'].includes(req.query.lang) ? req.query.lang : 'de';
     const limit  = Math.min(parseInt(req.query.limit) || 20, 100);
+    const userId = req.user.userId;
+    // Vom Client schon geladene Pairs (In-Session-Dedupe) – nur gültige UUIDs übernehmen.
+    const exclude = String(req.query.exclude || '')
+      .split(',')
+      .map(s => s.trim())
+      .filter(s => /^[0-9a-f-]{36}$/i.test(s));
 
     // 1. Random pairs — only fully ready content:
     //    pair published + linked question/statements published + a published picture exists.
     //    (Audio coverage is additionally enforced in Phase 2.)
+    //    Pagination: bereits abgeschlossene (user_pair_progress) und vom Client
+    //    geladene Pairs werden ausgeschlossen; leere Antwort = keine weiteren Karten.
     const pairsRes = await query(
       `SELECT p.id, p.answer_type, p.status, p.difficulty_level,
               p.question_id, p.positive_statement_id, p.negative_statement_id
@@ -61,9 +69,13 @@ router.get('/', requireJwt, async (req, res, next) => {
                   JOIN object_pictures pic ON pic.object_id = op.object_id
                   JOIN pictures pp ON pp.id = pic.picture_id
                  WHERE op.pair_id = p.id AND pp.status = 'published')
+          AND NOT EXISTS (
+                SELECT 1 FROM user_pair_progress upp
+                 WHERE upp.pair_id = p.id AND upp.user_id = $2)
+          AND p.id <> ALL($3::uuid[])
         ORDER BY random()
         LIMIT $1`,
-      [limit]
+      [limit, userId, exclude]
     );
     if (!pairsRes.rows.length) return res.json([]);
     const pairs   = pairsRes.rows;