Fix DeepL API auth: use header-based authentication

backend/api.py

@@ -163,8 +163,10 @@ async def translate_with_deepl(texts: List[str], target_lang: str) -> List[str]:
             async with httpx.AsyncClient() as client:
                 response = await client.post(
                     DEEPL_API_URL,
+                    headers={
+                        "Authorization": f"DeepL-Auth-Key {DEEPL_API_KEY}"
+                    },
                     data={
-                        "auth_key": DEEPL_API_KEY,
                         "text": texts_to_translate,
                         "target_lang": deepl_lang,
                         "source_lang": "EN"