feat: enhance database queries with error handling and improve SQL statement readability

Co-authored-by: Copilot <copilot@github.com>

backend/app/routers/auth.py

@@ -24,7 +24,8 @@ async def register(body: RegisterRequest) -> dict:
     try:
         user = await register_user(body.email, body.password)
     except ValueError as exc:
-        raise HTTPException(status_code=status.HTTP_409_CONFLICT, detail=str(exc))
+        raise HTTPException(
+            status_code=status.HTTP_409_CONFLICT, detail=str(exc))
     return {"id": user["id"], "email": user["email"], "role": user["role"]}
 
 
@@ -40,7 +41,8 @@ async def login(body: LoginRequest) -> TokenResponse:
     jti = str(uuid.uuid4())
     await store_refresh_token(user["id"], jti)
     return TokenResponse(
-        access_token=create_access_token(user["id"], user["email"], user["role"]),
+        access_token=create_access_token(
+            user["id"], user["email"], user["role"]),
         refresh_token=create_refresh_token(user["id"], jti),
     )
 
@@ -73,9 +75,8 @@ async def refresh(body: RefreshRequest) -> TokenResponse:
 
     from ..db import get_conn
     conn = get_conn()
-    row = conn.execute(
-        "SELECT email, role FROM users WHERE id = ?", [user_id]
-    ).fetchone()
+    sql_fetch = "SELECT email, role FROM users WHERE id = ?"
+    row = conn.execute(sql_fetch, [user_id]).fetchone()
     if row is None:
         raise credentials_error