Extract default proxy ports as named constants in config.go

[hanneshapke]

src/backend/config/config.go hardcodes the default proxy ports inline:

• Line 245: `ProxyPort: ":8080"` (forward proxy)
• Line 264: `ProxyPort: ":8081"` (transparent proxy)

These values are also referenced in README.md, docs/, and the frontend, so a magic literal in the config struct is easy to drift from documentation.

What to do

1. Add named constants near the top of config.go, e.g.:
   ```go
   const (
   DefaultForwardProxyPort = ":8080"
   DefaultTransparentProxyPort = ":8081"
   )
   ```
2. Use them in the default-config constructor instead of the inline literals.
3. (Optional) Reference the constants from any other place that hardcodes :8080 / :8081 in the Go code.

Hints

• Run grep -rn '8080\\|8081' src/backend to find other occurrences.
• Run make check to confirm no lint regressions.
• Don't touch docs/README in this issue — keep the change minimal.

Difficulty: good first issue, ~30 min.