Update alembic requirement from 1.14.1 to 1.15.1 in /backend (#1148)

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backend/Pipfile

@@ -7,7 +7,7 @@ name = "pypi"
 aiofiles = "24.1.0"
 aiohttp = "3.11.12"
 aiopg = "1.4.0"
-alembic = "1.14.1"
+alembic = "1.15.1"
 bcrypt = "4.3.0"
 click = "8.1.8"
 databases = {extras = ["asyncpg"], version = "<=0.8.0"}