Documentation: database requirements and local Postgres setup

[RagingRedRiot]

Background

The README's Running locally section assumes Postgres but doesn't pin a minimum version, and there's no scaffolded local-Postgres setup committed to the repo.

The schema requires PostgreSQL 18+: migrations/20260519000001_init.sql uses the built-in uuidv7() function as a column default on users, rooms, and messages. On Postgres 17 or earlier, migrations fail at startup (and on sqlx::test) with function uuidv7() does not exist — this is not a Postgres extension, it's a function added to core in PG18.

docker-compose.yml is gitignored, but a committed example would shorten the time from git clone to a passing cargo test.

Asks

• Document the PostgreSQL 18+ requirement in README Running locally. Make it impossible to miss.
• Add a docker-compose.yaml.example at the repo root that brings up a Postgres 18 instance matching the default DATABASE_URL in .env.example. Include a README pointer covering cp docker-compose.yaml.example docker-compose.yml and docker compose up -d.

Acceptance criteria

• A reader with no prior context can go from clone to passing cargo test using only the README and the example files.
• The Postgres version requirement is impossible to miss when reading setup instructions.