feat: add SQLAlchemy models and database setup

backend/app/database.py

@@ -0,0 +1,31 @@
+from sqlalchemy import create_engine
+from sqlalchemy.orm import DeclarativeBase, sessionmaker
+from pydantic_settings import BaseSettings
+
+
+class Settings(BaseSettings):
+    database_url: str
+    jwt_secret_key: str
+    jwt_algorithm: str = "HS256"
+    jwt_expire_minutes: int = 60
+
+    class Config:
+        env_file = ".env"
+
+
+settings = Settings()
+
+engine = create_engine(settings.database_url)
+SessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)
+
+
+class Base(DeclarativeBase):
+    pass
+
+
+def get_db():
+    db = SessionLocal()
+    try:
+        yield db
+    finally:
+        db.close()