feat(open-webui): add OpenWeb UI module for LLM interface

Add native NixOS module wrapping services.open-webui with:
- Configurable Ollama endpoint (defaults to 100.64.0.1:11434 via Headscale)
- SOPS secret management for WEBUI_SECRET_KEY
- Nginx reverse proxy integration with relaxed CSP for WebSockets
- Exposed at ai.ashisgreat.xyz

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

modules/default.nix

@@ -12,5 +12,6 @@
     ./adguard.nix
     ./forgejo.nix
     ./headscale.nix
+    ./open-webui.nix
   ];
 }