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>

configuration.nix

@@ -90,6 +90,11 @@
 
   nix.settings.experimental-features = [ "nix-command" "flakes" ];
 
+  # Allow unfree packages (for open-webui)
+  nixpkgs.config.allowUnfreePredicate = pkg: builtins.elem (lib.getName pkg) [
+    "open-webui"
+  ];
+
   # === SearXNG ===
   myModules.searxng = {
     enable = true;
@@ -203,6 +208,13 @@
   # === Tailscale Client ===
   services.tailscale.enable = true;
 
+  # === OpenWeb UI ===
+  myModules.open-webui = {
+    enable = true;
+    domain = "ai.ashisgreat.xyz";
+    ollamaUrl = "http://100.64.0.1:11434";
+  };
+
   # === Backups (Restic + B2) ===
   myModules.backup = {
     enable = true;