docs: add Netdata module design spec

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

docs/superpowers/specs/2026-03-21-netdata-design.md

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+# Netdata Module Design
+
+**Date:** 2026-03-21
+**Status:** Draft
+
+## Overview
+
+Add Netdata real-time monitoring to the NixOS VPS, accessible only from the Headscale/Tailscale network.
+
+## Requirements
+
+- Netdata monitoring service running on the VPS
+- Accessible via nginx reverse proxy with automatic HTTPS
+- Restricted to Tailscale network only (100.64.0.0/10) and localhost
+- Direct access on Tailscale IP (port 19999) also available
+
+## Implementation
+
+### Module: `modules/netdata.nix`
+
+Create a new module following the existing pattern.
+
+**Header comment block:**
+```nix
+# Netdata Module
+# Provides: Real-time system monitoring dashboard
+#
+# Usage:
+#   myModules.netdata = {
+#     enable = true;
+#     domain = "netdata.example.com";
+#   };
+#
+# Access is restricted to Tailscale network only via nginx internalOnly.
+```
+
+**Options:**
+- `enable` - Enable Netdata monitoring
+- `domain` - Domain for nginx reverse proxy (e.g., `netdata.ashisgreat.xyz`)
+- `port` - Internal port (default: 19999), description: "Internal port for Netdata to listen on"
+
+**Configuration:**
+- Enable `services.netdata` with default settings
+- Bind Netdata to `0.0.0.0` to allow direct Tailscale access (not just localhost)
+- Register domain with `myModules.nginx.domains` using `internalOnly = true`
+- Set `contentSecurityPolicy = null` - Netdata dashboard has its own CSP requirements
+- No firewall changes needed (nginx handles external access, direct Tailscale access works via mesh network)
+
+### Usage in configuration.nix
+
+```nix
+myModules.netdata = {
+  enable = true;
+  domain = "netdata.ashisgreat.xyz";
+};
+```
+
+### Access Control
+
+- **Via domain:** Only accessible from IPs in `100.64.0.0/10` (Tailscale) or `127.0.0.0/8` (localhost)
+- **Direct Tailscale:** `http://<tailscale-ip>:19999` (Tailscale mesh handles access control)
+
+### Backup Decision
+
+Netdata metrics data is **not backed up**. Rationale:
+- Metrics are ephemeral and regeneratable
+- Historical data is downsampled over time (not critical)
+- `/var/lib/netdata` excluded from backup paths
+
+### Secrets
+
+No SOPS secrets required. Netdata operates without authentication at the service level - access control is enforced via nginx/Tailscale network restrictions.
+
+## Files Changed
+
+| File | Action |
+|------|--------|
+| `modules/netdata.nix` | Create |
+| `modules/default.nix` | Add import |
+| `configuration.nix` | Enable module |
+
+## Security
+
+- No public internet access - blocked at nginx level
+- No authentication required at Netdata level (network-level access control)
+- Automatic HTTPS via Let's Encrypt

modules/open-webui-podman.nix

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+# OpenWebUI Podman Module
+# Provides: Web interface for LLMs using official Docker image
+#
+# Usage:
+#   myModules.open-webui-podman = {
+#     enable = true;
+#     port = 9000;
+#     domain = "ai.example.com";
+#     ollamaUrl = "http://100.64.0.1:11434";
+#   };
+
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+
+let
+  cfg = config.myModules.open-webui-podman;
+in
+{
+  options.myModules.open-webui-podman = {
+    enable = lib.mkEnableOption "OpenWebUI for LLMs via Podman";
+
+    port = lib.mkOption {
+      type = lib.types.port;
+      default = 9000;
+      description = "Port to expose OpenWebUI on localhost";
+    };
+
+    domain = lib.mkOption {
+      type = lib.types.str;
+      example = "ai.example.com";
+      description = "Public domain name for OpenWebUI";
+    };
+
+    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 {
+    # Ensure podman is enabled
+    myModules.podman.enable = true;
+
+    # Podman container for OpenWebUI
+    virtualisation.oci-containers.containers.open-webui = {
+      image = "ghcr.io/open-webui/open-webui:main";
+      ports = ["127.0.0.1:${toString cfg.port}:8080"];
+      environment = {
+        OLLAMA_API_BASE_URL = cfg.ollamaUrl;
+        WEBUI_URL = "https://${cfg.domain}";
+      };
+      environmentFiles = [config.sops.templates."openwebui-podman.env".path];
+      volumes = [
+        "open-webui-data:/app/backend/data"
+      ];
+    };
+
+    # SOPS template for secrets
+    sops.templates."openwebui-podman.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;
+      '';
+      # WebSocket support for /ws/
+      extraLocations."/ws/" = {
+        proxyPass = "http://127.0.0.1:${toString cfg.port}";
+        extraConfig = ''
+          proxy_http_version 1.1;
+          proxy_set_header Upgrade $http_upgrade;
+          proxy_set_header Connection "upgrade";
+          proxy_set_header Host $host;
+          proxy_read_timeout 86400;
+          proxy_send_timeout 86400;
+        '';
+      };
+      # Relaxed CSP for OpenWeb UI
+      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'";
+    };
+  };
+}