Base url additions and bug fixes (#519)

- Base URL option in settings for reverse proxy setups
- Fix NZB downloads not deleting on completion
- Fix handling for audiobook files over 100+ parts
- Fix prowlarr search timeout 
- Fix prowlarr categorisation for expanded searches

docs/reverse-proxy.md

@@ -0,0 +1,35 @@
+# Reverse Proxy & Subpath Hosting
+
+Shelfmark can run behind a reverse proxy at the root path (recommended) or
+under a subpath like `/shelfmark`.
+
+## Subpath setup
+
+1) Set the base path in Shelfmark:
+- UI: Settings → Advanced → Base Path
+- Env var: `URL_BASE=/shelfmark`
+
+2) Configure your reverse proxy to forward the subpath to Shelfmark and
+**strip the prefix** before sending to the backend. The proxy must also allow
+WebSocket upgrades for Socket.IO.
+
+Example (Nginx-style):
+
+```
+location /shelfmark/ {
+  proxy_pass http://shelfmark:8084/;
+  proxy_http_version 1.1;
+  proxy_set_header Upgrade $http_upgrade;
+  proxy_set_header Connection "upgrade";
+}
+```
+
+Notes:
+- Use a trailing slash on the `location` and `proxy_pass` to ensure the
+  `/shelfmark` prefix is removed.
+- Health checks still work at `/api/health` without the subpath.
+
+## Root path setup
+
+If you can serve Shelfmark at the root path (`https://shelfmark.example.com/`),
+leave `URL_BASE` empty. This is the simplest option.