feat: add src option for flake-compatible pure evaluation

When services.webapp.src is set, it is used directly instead of
builtins.fetchGit. This allows passing a flake input as the source,
which is required for pure evaluation mode in Nix flakes.

webapp.nix

@@ -9,8 +9,8 @@ let
     pname = "aio-webapp";
     version = "1.0.0";
 
-    # Fetch source from git repository
-    src = builtins.fetchGit {
+    # Use src option if provided (flake input), otherwise fetch from git
+    src = if cfg.src != null then cfg.src else builtins.fetchGit {
       url = cfg.gitUrl;
       ref = cfg.gitRef;
     };
@@ -88,6 +88,15 @@ in {
   options.services.webapp = {
     enable = lib.mkEnableOption "AIO webapp";
 
+    src = lib.mkOption {
+      type = lib.types.nullOr lib.types.path;
+      default = null;
+      description = ''
+        Source tree for the webapp. When set, this is used directly instead of
+        fetching via builtins.fetchGit. Use this with flake inputs for pure evaluation.
+      '';
+    };
+
     gitUrl = lib.mkOption {
       type = lib.types.str;
       default = "https://git.atitlan.io/aiolabs/webapp";