Use Pool instead of Client from postgres module

[Narigo]

Description
The backend uses a single Client for all queries to the database in a single shop. It could use the Pool and work with just a couple of connections for all shops of an instance.

Links/References
(Add any designs, screenshots or links that helpful to implement the feature)

Tasks

• Check that each shop still has its own prefix "connection"
• Use the same pool for the real database connection
• Test that everything still works with multiple connections

Additional Context
The current architecture should work for a couple of shops if there are not too many people reserving slots at the same time. Using a Pool instead would make it scale better if you decide to host multiple highly frequented shops.