collab: Setup database for LLM service (#15882)

This PR puts the initial infrastructure for the LLM service's database
in place.

The LLM service will be using a separate Postgres database, with its own
set of migrations.

Currently we only connect to the database in development, as we don't
yet have the database setup for the staging/production environments.

Release Notes:

- N/A

docker-compose.sql

@@ -1 +1,2 @@
 create database zed;
+create database zed_llm;