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

crates/collab/src/tests/test_server.rs

@@ -651,6 +651,9 @@ impl TestServer {
                 live_kit_server: None,
                 live_kit_key: None,
                 live_kit_secret: None,
+                llm_database_url: None,
+                llm_database_max_connections: None,
+                llm_database_migrations_path: None,
                 llm_api_secret: None,
                 rust_log: None,
                 log_json: None,