Use permanent storage for database.db

Follow-up PR after [#2](https://huggingface.co/spaces/ttseval/TTS-Arena/discussions/2) cc

@mrfakename


@hysts


With this PR, no need to re-download the database if the persistent storage is enabled. This should avoid loosing data because restarts. In case we disable permanent storage in the future (for any reason), the app will still work and default to the local storage as done at the moment.

app.py

@@ -14,7 +14,9 @@ SPACE_ID = os.getenv('HF_ID')
 
 DB_DATASET_ID = os.getenv('DATASET_ID')
 DB_NAME = "database.db"
-DB_PATH = "database.db"
+
+# If /data available => means local storage is enabled => let's use it!
+DB_PATH = f"/data/{DB_NAME}" if os.path.isdir("/data") else DB_NAME
 
 AUDIO_DATASET_ID = "ttseval/tts-arena-new"
 
@@ -79,14 +81,14 @@ def get_leaderboard():
 ####################################
 
 # Download existing DB
-print("Downloading DB...")
-try:
-    cache_path = hf_hub_download(repo_id=DB_DATASET_ID, repo_type='dataset', filename=DB_NAME)
-    shutil.copyfile(cache_path, DB_PATH)
-    print("Downloaded DB")
-except Exception as e:
-    print("Error while downloading DB:", e)
-
+if not os.path.isfile(DB_PATH):
+    print("Downloading DB...")
+    try:
+        cache_path = hf_hub_download(repo_id=DB_DATASET_ID, repo_type='dataset', filename=DB_NAME)
+        shutil.copyfile(cache_path, DB_PATH)
+        print("Downloaded DB")
+    except Exception as e:
+        print("Error while downloading DB:", e)
 
 # Create DB table (if doesn't exist)
 create_db_if_missing()