Add topic suggestions: search "banana" → suggests "Fruit/Obst" topic

31 curated topics with 683 words (Fruit, Animals, Body, Eye, Sports,
School, Family, Weather, etc.). When user types a word that belongs
to a topic, the topic appears as a suggestion with "Alle laden" button.

Clicking "Alle laden" fetches all words from that topic via Kaikki
and displays them for easy selection into a learning unit.

New endpoint: GET /api/vocabulary/topics?q=banana
New table: vocabulary_topics (topic, words[], word_count)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend-lehrer/vocabulary/api.py

@@ -431,6 +431,32 @@ async def api_enrich_images(word_ids: List[str] = None):
     return {"enriched": count, "total": len(word_ids)}
 
 
+@router.get("/topics")
+async def api_get_topics(q: str = Query("", description="Search topic or word")):
+    """Find topics matching a search word. Returns related word lists.
+
+    If q matches a topic name → returns that topic.
+    If q matches a word in any topic → returns all topics containing that word.
+    """
+    from vocabulary.db import get_pool
+    pool = await get_pool()
+    async with pool.acquire() as conn:
+        if not q.strip():
+            rows = await conn.fetch("SELECT topic, words, word_count FROM vocabulary_topics ORDER BY topic LIMIT 50")
+        else:
+            q_lower = q.strip().lower()
+            rows = await conn.fetch("""
+                SELECT topic, words, word_count FROM vocabulary_topics
+                WHERE lower(topic) LIKE $1 OR $2 = ANY(words)
+                ORDER BY word_count DESC
+            """, f"%{q_lower}%", q_lower)
+
+    return {
+        "topics": [{"topic": r["topic"], "words": list(r["words"]), "word_count": r["word_count"]} for r in rows],
+        "query": q,
+    }
+
+
 class TranslateRequest(BaseModel):
     word_ids: List[str]
     target_language: str