feat(db): add SQLite database engine and session management

backend/app/database.py

@@ -0,0 +1,23 @@
+from sqlalchemy import create_engine
+from sqlalchemy.orm import DeclarativeBase, sessionmaker
+
+DATABASE_URL = "sqlite:///./netmapper.db"
+
+engine = create_engine(
+    DATABASE_URL,
+    connect_args={"check_same_thread": False},
+)
+
+SessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)
+
+
+class Base(DeclarativeBase):
+    pass
+
+
+def get_db():
+    db = SessionLocal()
+    try:
+        yield db
+    finally:
+        db.close()