feat: use file-based SQLite database for persistent failure tracking

Dockerfile

@@ -4,6 +4,8 @@ ENV PYTHONUNBUFFERED=1
 
 WORKDIR /app
 
+RUN mkdir -p /app/data
+
 COPY requirements.txt .
 RUN pip install --no-cache-dir -r requirements.txt
 

main.py

@@ -81,7 +81,8 @@ db_conn = None
 
 def init_db():
     global db_conn
-    db_conn = sqlite3.connect(':memory:', check_same_thread=False)
+    os.makedirs('/app/data', exist_ok=True)
+    db_conn = sqlite3.connect('/app/data/submit_history.db', check_same_thread=False)
     db_conn.execute('''CREATE TABLE IF NOT EXISTS submitted (
         model_id TEXT, gpu TEXT, task_id TEXT, status TEXT,
         submitted_at TEXT, checked_at TEXT,