Add package and extensions options

- package: allows overriding the fava package
- extensions: list of Python packages to include as fava extensions
  (e.g., fava-dashboards). Automatically added to fava's Python
  environment via overridePythonAttrs.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

fava.nix

@@ -5,10 +5,30 @@ with lib;
 let
   cfg = config.services.fava;
 
+  # Build fava with extensions included in its Python environment
+  favaWithExtensions = if cfg.extensions == [] then cfg.package else
+    cfg.package.overridePythonAttrs (old: {
+      propagatedBuildInputs = (old.propagatedBuildInputs or []) ++ cfg.extensions;
+    });
+
 in {
   options.services.fava = {
     enable = mkEnableOption "Fava web interface for Beancount";
 
+    package = mkOption {
+      type = types.package;
+      default = pkgs.fava;
+      defaultText = literalExpression "pkgs.fava";
+      description = "The fava package to use";
+    };
+
+    extensions = mkOption {
+      type = types.listOf types.package;
+      default = [];
+      description = "Python packages to include as fava extensions";
+      example = literalExpression "[ pkgs.python3Packages.fava-dashboards ]";
+    };
+
     ledgerFile = mkOption {
       type = types.path;
       default = "/var/lib/fava/ledger.beancount";
@@ -103,7 +123,7 @@ in {
         Group = cfg.group;
         WorkingDirectory = cfg.dataDir;
 
-        ExecStart = "${pkgs.fava}/bin/fava ${cfg.ledgerFile} --host ${cfg.host} --port ${toString cfg.port} ${concatStringsSep " " cfg.extraOptions}";
+        ExecStart = "${favaWithExtensions}/bin/fava ${cfg.ledgerFile} --host ${cfg.host} --port ${toString cfg.port} ${concatStringsSep " " cfg.extraOptions}";
 
         Restart = "on-failure";
         RestartSec = "5s";