Add daemon mode for background checks with systemd support

- Added daemon command with --interval, --pid-file, --since flags
- Configurable check interval (s/m/h/d format)
- PID file for status checking
- Graceful shutdown on SIGTERM/SIGINT
- Structured logging to stderr for systemd journal
- Updated README with daemon docs and systemd service template
- Added daemon tests (parse_interval, PID file, help)

README.md

@@ -12,6 +12,7 @@ Some repositories (e.g., Zammad) only use git tags without creating GitHub relea
 - Built-in HTTP server for local feed access
 - Rate-limit-aware GitHub API client
 - Configurable via environment variables and CLI flags
+- Daemon mode for automatic background checks
 
 ## Installation
 
@@ -60,6 +61,7 @@ ghrel                          # Main command
 ├── update <owner/repo> [--tags] [--release]  # Update tracking mode
 ├── list                       # List all tracked repositories
 ├── check [--since N]          # Fetch latest releases/tags
+├── daemon [--interval 30m]    # Run as background daemon
 └── serve [--port 8080] [--host 127.0.0.1]  # Start HTTP server
 ```
 
@@ -117,6 +119,56 @@ ghrel serve                        # Default: 127.0.0.1:8080
 ghrel serve --port 9090 --host 0.0.0.0  # Custom port and host
 ```
 
+### `daemon [--interval INTERVAL] [--pid-file PATH] [--since N]`
+
+Run as a background daemon that periodically checks all tracked repositories for new releases or tags.
+
+```bash
+ghrel daemon                      # Default: 30m interval
+ghrel daemon --interval 1h        # Check every hour
+ghrel daemon --interval 15m       # Check every 15 minutes
+ghrel daemon --pid-file /run/ghrel.pid  # Custom PID file
+ghrel daemon --since 24h          # Only entries from last 24h
+```
+
+**Interval format**: `N` followed by `s` (seconds), `m` (minutes), `h` (hours), or `d` (days).
+
+The daemon writes a PID file for status checking and sends structured logs to stderr (suitable for systemd journal). It handles `SIGTERM` and `SIGINT` for graceful shutdown.
+
+#### Systemd Service
+
+Create `/etc/systemd/system/ghrel-daemon.service`:
+
+```ini
+[Unit]
+Description=GitHub Release Monitor Daemon
+After=network.target
+
+[Service]
+Type=simple
+ExecStart=/usr/local/bin/ghrel daemon --interval 30m --pid-file /run/ghrel.pid
+Restart=on-failure
+RestartSec=10
+Environment=GH_TOKEN=your_token_here
+
+[Install]
+WantedBy=multi-user.target
+```
+
+Enable and start:
+
+```bash
+sudo systemctl daemon-reload
+sudo systemctl enable --now ghrel-daemon
+sudo systemctl status ghrel-daemon
+```
+
+View logs:
+```bash
+journalctl -u ghrel-daemon -f
+```
+```
+
 ## Configuration
 
 | Setting       | Default                    | Override          |