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

Dockerfile

@@ -27,5 +27,7 @@ RUN apt-get update; \
 WORKDIR app
 COPY --from=builder /app/collab /app/collab
 COPY --from=builder /app/crates/collab/migrations /app/migrations
+COPY --from=builder /app/crates/collab/migrations_llm /app/migrations_llm
 ENV MIGRATIONS_PATH=/app/migrations
+ENV LLM_DATABASE_MIGRATIONS_PATH=/app/migrations_llm
 ENTRYPOINT ["/app/collab"]