dev setup: allow Postgres and Redis host ports to be configured

## Summary

`just setup` requires host ports `5432` and `6379` for its Docker-managed Postgres and Redis services. This blocks local setup when standard Homebrew Postgres or Redis services are already running, even though Buzz itself supports custom `DATABASE_URL` and `REDIS_URL` values.

## Environment

- Buzz `main` at `acfbb1bb`
- macOS
- Docker Desktop 29.3.1 / Docker Compose 5.1.1
- Homebrew `postgresql@15` on `localhost:5432`
- Homebrew `redis` on `localhost:6379`

## Reproduction

1. Start local Postgres and Redis services on their standard ports.
2. Clone Buzz and activate Hermit.
3. Run:

```bash
just setup
```

## Actual behavior

Setup aborts before Compose starts:

```text
[dev-setup] Local Redis is already listening on port 6379 (...).
Stop it before running setup: brew services stop redis
```

Postgres would also conflict because `docker-compose.yml` publishes `5432:5432`.

The fixed assumptions are currently spread across:

- `docker-compose.yml`: `5432:5432` and `6379:6379`
- `scripts/dev-setup.sh`: collision detection always checks `6379`
- `.env.example`: URLs and `PGPORT` use the standard ports

Stopping existing database services can disrupt unrelated local applications.

## Expected behavior

Buzz should preserve the current defaults while allowing its Docker services to bind configurable host ports, for example:

```dotenv
DATABASE_URL=postgres://buzz:buzz_dev@localhost:55432/buzz
PGPORT=55432
BUZZ_REDIS_HOST_PORT=56379
REDIS_URL=redis://localhost:56379
```

Then `just setup`, `just relay`, and `just dev` should use those values without requiring existing services to stop.

## Proposed direction

- Publish Postgres as `${PGPORT:-5432}:5432`.
- Add a Redis host-port setting, e.g. `BUZZ_REDIS_HOST_PORT`, and publish `${BUZZ_REDIS_HOST_PORT:-6379}:6379`.
- Make the local Redis collision check inspect the configured host port.
- Document that each host-port value and its corresponding connection URL must be changed together.
- Consider making integration-test scripts honor the configured URLs/ports as a follow-up.

## Local verification

I tested the proposed approach with Postgres on `55432` and Redis on `56379`:

- `just setup` completed.
- Database migrations and local community seeding completed.
- Both Buzz containers reported healthy.
- Both existing Homebrew services remained running.
- Redis returned `PONG` on both `6379` (Homebrew) and `56379` (Buzz).