fix(docker-compose): make web host port overridable for macOS (#37)

[ChrisJr404]

Closes #37.

macOS Monterey+ reserves port 5000 for Control Center (AirPlay Receiver / Handoff), so `docker-compose up` fails out-of-the-box on Mac with `Ports are not available: ... bind: address already in use`. Mac users either had to disable AirPlay system-wide or hand-edit the compose file.

Replaces the hard-coded `5000:5000` mapping with `${WEB_PORT:-5000}:5000`. Default stays `5000` so existing Linux setups are completely unchanged; Mac users sidestep the conflict with one env var:

```bash
WEB_PORT=8085 docker-compose up -d --build
```

The README's run section now has a callout explaining the AirPlay conflict + the override, so Mac users don't have to dig through issue history.

I went with the env-var approach instead of the issue's suggested "just change the default to 8085" because:

• Linux + WSL users keep the same port they've always used. Zero docs/curl/walkthrough breakage.
• All existing references in README to `http://localhost:5000\` (API docs, GraphQL endpoint, merchant integration examples, the upload PoC commands) stay correct.
• One env var works for any other macOS host that happens to have something else on 5000 too — not just Control Center.

`docker compose config` round-trips correctly with the override:

```
$ WEB_PORT=8085 docker compose config | grep -A 1 published
published: "5432" # postgres, unchanged
published: "8085" # web — picked up the env var
published: "80" # alternate web mapping, unchanged
```

Test plan:

• `docker compose config` syntactically clean
• Env var override resolves to the supplied port
• Default (no env var) still resolves to 5000
• README callout clearly distinguishes the macOS path from the Linux/WSL path