Add missing relation endpoints for content_mentor migration

- GET /api/objects?picture_id=X filter via object_pictures join
- GET /api/objects/:id/words — full word details
- GET /api/objects/:id/pairs — pairs with nested question + statements
- GET /api/words?titel_de=X case-insensitive filter
- GET /api/pictures/:id/words — full word details
- DELETE /api/pictures/:id/words/:wordId — convenience unlink

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

src/routes/words.js

@@ -12,10 +12,12 @@ const STATUS_TIMESTAMP = {
 // GET /api/words
 router.get('/', async (req, res, next) => {
   try {
-    const { status, limit = 50, offset = 0 } = req.query;
+    const { status, titel_de, limit = 50, offset = 0 } = req.query;
     const params = [Math.min(parseInt(limit), 500), parseInt(offset)];
-    const where = status ? `WHERE w.status = $3` : '';
-    if (status) params.push(status);
+    const conditions = [];
+    if (status) { conditions.push(`w.status = $${params.length + 1}`); params.push(status); }
+    if (titel_de) { conditions.push(`lower(w.titel_de) = lower($${params.length + 1})`); params.push(titel_de); }
+    const where = conditions.length ? `WHERE ${conditions.join(' AND ')}` : '';
     const result = await query(
       `SELECT w.*,
         COALESCE(json_agg(DISTINCT p.id) FILTER (WHERE p.id IS NOT NULL), '[]') AS picture_ids,