Implement HTTP timeout configurations and enhance API documentation

- Added optional HTTP server timeout configurations (`HNHMAP_READ_TIMEOUT`, `HNHMAP_WRITE_TIMEOUT`, `HNHMAP_IDLE_TIMEOUT`) to `.env.example` and updated the server initialization in `main.go` to utilize these settings.
- Enhanced API documentation for the `rebuildZooms` endpoint to clarify its background processing and polling mechanism for status updates.
- Updated `configuration.md` to include new timeout environment variables for better configuration guidance.
- Improved error handling in the client for large request bodies, ensuring appropriate responses for oversized payloads.

docs/configuration.md

@@ -6,6 +6,9 @@
 |-----------------|-------------|---------|
 | `HNHMAP_PORT` | HTTP server port | 8080 |
 | `-port` | Same (command-line flag) | value of `HNHMAP_PORT` or 8080 |
+| `HNHMAP_READ_TIMEOUT` | HTTP server read timeout (e.g. `30s`) | 30s |
+| `HNHMAP_WRITE_TIMEOUT` | HTTP server write timeout (e.g. `60s`) | 60s |
+| `HNHMAP_IDLE_TIMEOUT` | HTTP server idle timeout (e.g. `120s`) | 120s |
 | `HNHMAP_BOOTSTRAP_PASSWORD` | Password for initial setup: when no users exist, logging in as `admin` with this password creates the first admin user | — |
 | `HNHMAP_BASE_URL` | Full application URL for OAuth redirect_uri (e.g. `https://map.example.com`). If not set, derived from `Host` and `X-Forwarded-*` headers | — |
 | `HNHMAP_OAUTH_GOOGLE_CLIENT_ID` | Google OAuth Client ID | — |