fix(tenancy): use is_production() for invitation URL instead of debug flag

Using debug flag for environment detection is unreliable — if left
True in prod, links would point to localhost. Now uses the proper
is_production() from environment module.

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

static/shared/js/api-client.js

@@ -207,16 +207,15 @@ class APIClient {
     /**
      * POST request
      */
-    async post(endpoint, data = {}) {
+    async post(endpoint, data = null) {
         apiLog.debug('POST request data:', {
             hasData: !!data,
-            dataKeys: Object.keys(data)
+            dataKeys: data ? Object.keys(data) : []
         });
 
-        return this.request(endpoint, {
-            method: 'POST',
-            body: JSON.stringify(data)
-        });
+        var opts = { method: 'POST' };
+        if (data != null) opts.body = JSON.stringify(data);
+        return this.request(endpoint, opts);
     }
 
     /**