security: add connectionStringFile option for PostgreSQL secrets

The connectionString option stores credentials in the world-readable
Nix store. This adds connectionStringFile as an alternative that reads
the connection string from a file at runtime, compatible with secret
management tools like agenix or sops-nix.

Changes:
- Add database.connectionStringFile option (mutually exclusive with connectionString)
- Read connection string from file at service start when configured
- Add warning to connectionString documentation about Nix store visibility
- Update README with examples for both approaches

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

README.md

@@ -181,7 +181,8 @@ A NixOS module is provided for running the MCP server as a systemd service.
 | `package` | package | from flake | Package to use |
 | `database.type` | enum | `"sqlite"` | `"sqlite"` or `"postgres"` |
 | `database.name` | string | `"nixos-options.db"` | SQLite database filename |
-| `database.connectionString` | string | `""` | PostgreSQL connection URL |
+| `database.connectionString` | string | `""` | PostgreSQL connection URL (stored in Nix store) |
+| `database.connectionStringFile` | path | `null` | Path to file with PostgreSQL connection URL (recommended for secrets) |
 | `indexOnStart` | list of string | `[]` | Revisions to index on service start |
 | `user` | string | `"nixos-options-mcp"` | User to run the service as |
 | `group` | string | `"nixos-options-mcp"` | Group to run the service as |
@@ -189,6 +190,8 @@ A NixOS module is provided for running the MCP server as a systemd service.
 
 ### PostgreSQL Example
 
+Using `connectionString` (stored in Nix store - suitable for testing or non-sensitive setups):
+
 ```nix
 {
   services.nixos-options-mcp = {
@@ -202,6 +205,25 @@ A NixOS module is provided for running the MCP server as a systemd service.
 }
 ```
 
+Using `connectionStringFile` (recommended for production with sensitive credentials):
+
+```nix
+{
+  services.nixos-options-mcp = {
+    enable = true;
+    database = {
+      type = "postgres";
+      # File contains: postgres://user:secret@localhost/nixos_options?sslmode=disable
+      connectionStringFile = "/run/secrets/nixos-options-db";
+    };
+    indexOnStart = [ "nixos-unstable" ];
+  };
+
+  # Example with agenix or sops-nix for secret management
+  # age.secrets.nixos-options-db.file = ./secrets/nixos-options-db.age;
+}
+```
+
 ## Development
 
 ```bash

nix/module.nix

@@ -3,8 +3,14 @@
 let
   cfg = config.services.nixos-options-mcp;
 
+  # Determine database URL based on configuration
+  # For postgres with connectionStringFile, the URL is set at runtime via script
+  useConnectionStringFile = cfg.database.type == "postgres" && cfg.database.connectionStringFile != null;
+
   databaseUrl = if cfg.database.type == "sqlite"
     then "sqlite://${cfg.dataDir}/${cfg.database.name}"
+    else if useConnectionStringFile
+    then "" # Will be set at runtime from file
     else cfg.database.connectionString;
 in
 {
@@ -50,8 +56,26 @@ in
         description = ''
           PostgreSQL connection string (when using postgres backend).
           Example: "postgres://user:password@localhost/nixos_options?sslmode=disable"
+
+          WARNING: This value will be stored in the Nix store, which is world-readable.
+          For production use with sensitive credentials, use connectionStringFile instead.
         '';
       };
+
+      connectionStringFile = lib.mkOption {
+        type = lib.types.nullOr lib.types.path;
+        default = null;
+        description = ''
+          Path to a file containing the PostgreSQL connection string.
+          The file should contain just the connection string, e.g.:
+          postgres://user:password@localhost/nixos_options?sslmode=disable
+
+          This is the recommended way to configure PostgreSQL credentials
+          as the file is not stored in the world-readable Nix store.
+          The file must be readable by the service user.
+        '';
+        example = "/run/secrets/nixos-options-mcp-db";
+      };
     };
 
     indexOnStart = lib.mkOption {
@@ -78,8 +102,14 @@ in
   config = lib.mkIf cfg.enable {
     assertions = [
       {
-        assertion = cfg.database.type == "sqlite" || cfg.database.connectionString != "";
+        assertion = cfg.database.type == "sqlite"
-        message = "services.nixos-options-mcp.database.connectionString must be set when using postgres backend";
+          || cfg.database.connectionString != ""
+          || cfg.database.connectionStringFile != null;
+        message = "services.nixos-options-mcp.database: when using postgres backend, either connectionString or connectionStringFile must be set";
+      }
+      {
+        assertion = cfg.database.connectionString == "" || cfg.database.connectionStringFile == null;
+        message = "services.nixos-options-mcp.database: connectionString and connectionStringFile are mutually exclusive";
       }
     ];
 
@@ -102,22 +132,39 @@ in
       after = [ "network.target" ]
         ++ lib.optional (cfg.database.type == "postgres") "postgresql.service";
 
-      environment = {
+      environment = lib.mkIf (!useConnectionStringFile) {
         NIXOS_OPTIONS_DATABASE = databaseUrl;
       };
 
-      preStart = lib.mkIf (cfg.indexOnStart != [ ]) ''
+      path = [ cfg.package ];
+
+      script = let
+        indexCommands = lib.optionalString (cfg.indexOnStart != []) ''
           ${lib.concatMapStringsSep "\n" (rev: ''
             echo "Indexing revision: ${rev}"
-          ${cfg.package}/bin/nixos-options index "${rev}" || true
+            nixos-options index "${rev}" || true
           '') cfg.indexOnStart}
         '';
+      in
+        if useConnectionStringFile then ''
+          # Read database connection string from file
+          if [ ! -f "${cfg.database.connectionStringFile}" ]; then
+            echo "Error: connectionStringFile not found: ${cfg.database.connectionStringFile}" >&2
+            exit 1
+          fi
+          export NIXOS_OPTIONS_DATABASE="$(cat "${cfg.database.connectionStringFile}")"
+
+          ${indexCommands}
+          exec nixos-options serve
+        '' else ''
+          ${indexCommands}
+          exec nixos-options serve
+        '';
 
       serviceConfig = {
         Type = "simple";
         User = cfg.user;
         Group = cfg.group;
-        ExecStart = "${cfg.package}/bin/nixos-options serve";
         Restart = "on-failure";
         RestartSec = "5s";