feat: bring ts-bookstack up to current template spec

- Add SECURITY.md (matches template)
- Add tailscale/config/serve.headscale.json (HTTP-only variant)
- Default serve.json to Tailscale generic with ${TS_CERT_DOMAIN}
- Use linuxserver env var names (DB_USERNAME not DB_USER)
- Use named Docker volumes for cross-platform portability
- Both db and app use network_mode: service:bookstack-ts
- Rewrite README with Tailscale vs Headscale structure
- Rename db service to bookstack-db to avoid collisions with other ts-* repos

SECURITY.md

@@ -0,0 +1,30 @@
+# Security Notes
+
+## Sensitive Files
+
+- `.env` — Contains your Tailscale/Headscale auth key and service configuration. **Never commit this file.**
+- `tailscale/tailscale-data/` — Contains Tailscale node state and keys. **Never commit this directory.**
+
+## Capabilities
+
+This compose file adds the following capabilities to the Tailscale sidecar:
+
+- `net_admin` — Required for Tailscale to manage network interfaces and routes.
+
+These capabilities are necessary for Tailscale to function but increase the attack surface if the container is compromised.
+
+## Default Passwords & Secrets
+
+The `docker-compose.yml` and `.env` files contain placeholder values (`***`) for sensitive items such as:
+
+- Database credentials
+- Service JWT secret
+
+**Change these values** before deploying anywhere security matters. Never use the default placeholders in production or shared environments.
+
+## Recommendations
+
+- Use a dedicated, limited-scope auth key for each service.
+- Regularly rotate auth keys.
+- Review Headscale/Tailscale ACLs to ensure services only have the access they need.
+- Keep the Tailscale Docker image reasonably up to date.