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/open-webui.nix

@@ -0,0 +1,76 @@
+# OpenWeb UI Module
+# Provides: Web interface for LLMs (Ollama, OpenAI-compatible APIs)
+#
+# Usage:
+#   myModules.open-webui = {
+#     enable = true;
+#     port = 8080;
+#     domain = "ai.example.com";
+#     ollamaUrl = "http://100.64.0.1:11434";  # Remote Ollama via Tailscale/Headscale
+#   };
+
+{
+  config,
+  lib,
+  ...
+}:
+
+let
+  cfg = config.myModules.open-webui;
+in
+{
+  options.myModules.open-webui = {
+    enable = lib.mkEnableOption "OpenWeb UI for LLMs";
+
+    port = lib.mkOption {
+      type = lib.types.port;
+      default = 8080;
+      description = "Port to expose OpenWeb UI on localhost";
+    };
+
+    domain = lib.mkOption {
+      type = lib.types.str;
+      example = "ai.example.com";
+      description = "Public domain name for OpenWeb UI";
+    };
+
+    ollamaUrl = lib.mkOption {
+      type = lib.types.str;
+      default = "http://127.0.0.1:11434";
+      example = "http://100.64.0.1:11434";
+      description = "URL of the Ollama API endpoint";
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    services.open-webui = {
+      enable = true;
+      port = cfg.port;
+      host = "127.0.0.1";
+      environment = {
+        OLLAMA_API_BASE_URL = cfg.ollamaUrl;
+        WEBUI_URL = "https://${cfg.domain}";
+      };
+      environmentFile = config.sops.templates."openwebui.env".path;
+    };
+
+    # SOPS template for secrets
+    sops.templates."openwebui.env" = {
+      content = ''
+        WEBUI_SECRET_KEY=${config.sops.placeholder.openwebui_secret_key}
+      '';
+    };
+
+    sops.secrets.openwebui_secret_key = { };
+
+    # Nginx configuration
+    myModules.nginx.domains.${cfg.domain} = {
+      port = cfg.port;
+      extraConfig = ''
+        client_max_body_size 100M;
+      '';
+      # Relaxed CSP for OpenWeb UI — needs unsafe-eval for some JS, WebSockets, external images
+      contentSecurityPolicy = "default-src 'self'; script-src 'self' 'unsafe-inline' 'unsafe-eval'; style-src 'self' 'unsafe-inline'; img-src 'self' data: https:; font-src 'self' data:; connect-src 'self' wss: https:; frame-ancestors 'self'";
+    };
+  };
+}