docs: add OpenWeb UI module design spec

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

docs/superpowers/specs/2026-03-20-openwebui-design.md

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+# OpenWeb UI Module Design
+
+## Overview
+Add OpenWeb UI as a native NixOS service, connecting to Ollama running on the user's PC via Headscale at `http://100.64.0.1:11434`.
+
+## Architecture
+
+A thin wrapper module around the native `services.open-webui` NixOS module that:
+1. Configures connection to the remote Ollama instance
+2. Integrates with the existing nginx module for HTTPS
+3. Follows established module patterns (vaultwarden, forgejo)
+
+## Module Options
+
+```nix
+myModules.open-webui = {
+  enable = true;
+  port = 8080;
+  domain = "ai.ashisgreat.xyz";
+  ollamaUrl = "http://100.64.0.1:11434";
+};
+```
+
+## Components
+
+### 1. Module File: `modules/open-webui.nix`
+
+```nix
+{
+  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;
+      '';
+      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'";
+    };
+  };
+}
+```
+
+### 2. Update `modules/default.nix`
+
+Add `./open-webui.nix` to imports.
+
+### 3. Update `configuration.nix`
+
+```nix
+myModules.open-webui = {
+  enable = true;
+  domain = "ai.ashisgreat.xyz";
+  ollamaUrl = "http://100.64.0.1:11434";
+};
+```
+
+### 4. Add Secret to `secrets/secrets.yaml`
+
+```yaml
+openwebui_secret_key: <generated-secret>
+```
+
+## Files Changed
+
+| File | Change |
+|------|--------|
+| `modules/open-webui.nix` | New module |
+| `modules/default.nix` | Add import |
+| `configuration.nix` | Enable and configure |
+| `secrets/secrets.yaml` | Add `openwebui_secret_key` |
+
+## Security Considerations
+
+- Service binds to `127.0.0.1` only (nginx handles external access)
+- `WEBUI_SECRET_KEY` stored encrypted in SOPS
+- CSP headers configured for WebSocket support
+- Large upload limit (100M) for model files if needed
+
+## Dependencies
+
+- `services.open-webui` (available in nixpkgs unstable)
+- Existing nginx and SOPS infrastructure