feat(6.1-01): add second_auth_user fixture to conftest.py

- Add @pytest_asyncio.fixture second_auth_user with handle prefix 'user2_'
- Creates User + Quota row following the same pattern as auth_user
- Returns {user, token, headers} dict shape for use in sharing tests

backend/tests/conftest.py

@@ -226,6 +226,45 @@ async def auth_user(db_session: AsyncSession):
     }
 
 
+@pytest_asyncio.fixture
+async def second_auth_user(db_session: AsyncSession):
+    """Create a second regular user with a Quota row and return auth context.
+
+    Returns the same dict shape as auth_user but with a distinct handle prefix
+    ("user2_") so sharing tests can have a sharer and a recipient in the same
+    test without handle collisions.
+    """
+    import uuid as _uuid
+    from db.models import User, Quota
+    from services.auth import hash_password, create_access_token
+
+    user_id = _uuid.uuid4()
+    user = User(
+        id=user_id,
+        handle=f"user2_{user_id.hex[:8]}",
+        email=f"user2_{user_id.hex[:8]}@example.com",
+        password_hash=hash_password("Testpassword123!"),
+        role="user",
+        is_active=True,
+        password_must_change=False,
+    )
+    quota = Quota(
+        user_id=user_id,
+        limit_bytes=104857600,  # 100 MB
+        used_bytes=0,
+    )
+    db_session.add(user)
+    db_session.add(quota)
+    await db_session.commit()
+
+    token = create_access_token(str(user_id), "user")
+    return {
+        "user": user,
+        "token": token,
+        "headers": {"Authorization": f"Bearer {token}"},
+    }
+
+
 @pytest_asyncio.fixture
 async def admin_user(db_session: AsyncSession):
     """Create an admin user with a Quota row and return auth context.