feat: seed SSH users from env var and authorized_keys file (#31)

## Summary

Docker onboarding was broken — no way to add first SSH user without `docker attach` to TUI.

Now reads SSH public keys from two sources on startup:
- `UPTOP_ADMIN_KEY` env var — single key for quick single-user setup
- `UPTOP_KEYS` file path — authorized_keys format for team setup

Dockerfile already sets `UPTOP_KEYS=/data/authorized_keys` and compose mounts `./data:/data`, so the flow is:

```
echo "ssh-ed25519 AAAA... me@host" > ./data/authorized_keys
docker compose up -d
ssh -p 23234 localhost
```

### Behavior
- Skips keys already in DB (idempotent across restarts)
- All seeded users get admin role
- Username parsed from key comment (e.g. `tyler@macbook` → `tyler`)
- Comments and blank lines in keys file are ignored

### Tested
- UPTOP_ADMIN_KEY seeds single admin user
- UPTOP_KEYS file seeds multiple users with correct usernames
- Second startup skips existing keys (no duplicates)
- Build and all tests pass

Reviewed-on: lerko/uptop#31

docker-compose.yml

@@ -14,5 +14,7 @@ services:
       - UPTOP_HTTP_PORT=8080
       - UPTOP_STATUS_ENABLED=true
       - UPTOP_STATUS_TITLE=System Status
+      # SSH access: add your public key via env var or authorized_keys file
+      # - UPTOP_ADMIN_KEY=ssh-ed25519 AAAA... you@host
     volumes:
       - ./data:/data