refactor: integrate Alembic, restructure project, and clean up

- Add Alembic configuration (alembic.ini, env.py) for database migrations
- Remove legacy wheel.db and unnecessary __pycache__ files
- Delete models.py to separate SQLAlchemy and Pydantic concerns
- Restructure main.py to use modular API routers (user/admin)
- Introduce config backend for centralized settings management

backend/app/db/session.py

@@ -0,0 +1,27 @@
+from sqlalchemy import create_engine
+from sqlalchemy.ext.declarative import declarative_base
+from sqlalchemy.orm import sessionmaker
+
+DATABASE_URL = "sqlite:///./wheel.db"
+
+engine = create_engine(
+    DATABASE_URL,
+    connect_args={"check_same_thread": False}
+)
+
+SessionLocal = sessionmaker(
+    autocommit=False,
+    autoflush=False,
+    bind=engine
+)
+
+# Database Session
+def get_db():
+    db = SessionLocal()
+
+    try:
+        yield db
+    finally:
+        db.close()
+        
+Base = declarative_base()