(refactor) Add native systemd production deployment

Make native systemd services the recommended production path for
pobsync while keeping Docker Compose available for development and
optional test installs.

Add web, worker, and scheduler systemd unit templates, a native
environment example, an optional nginx reverse proxy template, and an
installer that creates the venv, service user, env file, units, and
runs migrations/static collection.

Allow native deployments to configure POBSYNC_BACKUP_ROOT directly and
document the new production layout and update flow.

deploy/nginx/pobsync.conf

@@ -0,0 +1,15 @@
+server {
+    listen 80;
+    server_name @POBSYNC_SERVER_NAME@;
+
+    client_max_body_size 16m;
+
+    location / {
+        proxy_pass http://127.0.0.1:8010;
+        proxy_http_version 1.1;
+        proxy_set_header Host $host;
+        proxy_set_header X-Real-IP $remote_addr;
+        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+        proxy_set_header X-Forwarded-Proto $scheme;
+    }
+}