Use postgres as default session service

[falquaddoomi]

As mentioned in #45, a recent(?) change to the version of google-adk we're using is causing an exception when we attempt to add figures to the context when using the default session service, InMemorySessionService.

A quick fix introduced in this PR is to switch to DatabaseSessionService, in our case backed by postgres. Recent(?) changes in google-adk require the database connection to be async-compatible, so this PR also adds a dependency on asyncpg and has the backend use it when constructing a database connection.

This PR amends the documentation to use the docker-compose.with-pg.yml compose config in both the dev and prod instructions, which starts a postgres container and instructs the backend to use it.

I also included a run_stack.sh shell script and would like to know if we'd like to switch the documentation over to using it. There are a few perks to using it:

• it copies the .env.TEMPLATE file to .env and fills in missing passwords with secure randomly-generated ones
• it constructs the required compose configs based on the selected environment, reducing possible mistakes
• IMHO it improves the behavior after launching the stack: dev remains attached, and prod detaches + tails the logs

Of course, encouraging them to use run_stack.sh implies they have a GNU environment in which to run bash scripts.

Closes #45.

[d33bs]

LGTM!

[d33bs]

This could be worth a quick check with shellcheck. I don't see anything especially dangerous but it might provide some good feedback for string composition.