feat: add systemd install script for daemon-only deployment

The daemon already handles bidirectional sync (SVN→Git polling and
Git→SVN push), so a single systemd service is sufficient. The script
creates a venv, generates svn-mirror.service, and enables it for a
given user (default: gitea). README updated accordingly.

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

README.md

@@ -235,14 +235,57 @@ it fetches the new commits from Gitea and runs `sync_git_to_svn`.
 - Secret: same as `webhook_secret` in config
 - Events: "Push"
 
-### Mode C: Both (recommended)
+### Mode C: systemd service (recommended)
 
-Run daemon AND webhook. The daemon handles SVN→Git polling; the webhook
-handles instant Git→SVN on developer push.
+The daemon handles **bidirectional** sync: SVN→Git polling AND Git→SVN push
+on each cycle. One service is all you need.
+
+#### Quick install via script
+
+Use the provided install script to generate and enable the systemd service
+running as a given user (e.g. `gitea`):
 
 ```bash
-screen -dmS daemon python3.11 -m svn_mirror daemon --config /etc/svn-mirror/config.yml
-screen -dmS webhook python3.11 -m svn_mirror webhook --config /etc/svn-mirror/config.yml
+sudo ./deploy/install.sh --install-dir /opt/svn-git-server --user gitea
+```
+
+This will:
+- Create a Python virtualenv at `<install-dir>/.venv` and install deps
+- Generate `svn-mirror.service`
+- Enable the service (auto-start on boot)
+
+Options:
+
+| Option | Default | Description |
+|--------|---------|-------------|
+| `--install-dir DIR` | *(required)* | Where the code is installed |
+| `--user USER` | `gitea` | System user to run the service as |
+| `--config FILE` | `<install-dir>/config.yml` | Config file path |
+| `--venv DIR` | `<install-dir>/.venv` | Virtualenv path |
+| `--uninstall` | — | Remove service instead of installing |
+
+After install, start the service:
+
+```bash
+sudo systemctl start svn-mirror
+```
+
+Check logs:
+
+```bash
+journalctl -u svn-mirror -f
+```
+
+To uninstall:
+
+```bash
+sudo ./deploy/install.sh --uninstall
+```
+
+#### Manual setup (screen/tmux)
+
+```bash
+screen -dmS svn-mirror python3.11 -m svn_mirror daemon --config /etc/svn-mirror/config.yml
 ```
 
 ### Mode D: Gitea post-receive hook (filesystem)