fix(tenant): Defer plugin seeding until after transaction commits

The post_save signal was trying to seed plugins before the tenant's
schema migrations had completed, causing a 500 error when accepting
tenant invitations. Using transaction.on_commit() ensures the schema
and tables exist before seeding.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

smoothschedule/core/signals.py

@@ -4,36 +4,26 @@ Core App Signals
 Handles automatic setup tasks when tenants are created.
 """
 import logging
+from django.db import transaction
 from django.db.models.signals import post_save
 from django.dispatch import receiver
 
 logger = logging.getLogger(__name__)
 
 
-@receiver(post_save, sender='core.Tenant')
-def seed_platform_plugins_on_tenant_create(sender, instance, created, **kwargs):
+def _seed_plugins_for_tenant(tenant_schema_name):
     """
-    Seed platform plugins when a new tenant is created.
-
-    This ensures new tenants have access to all marketplace plugins immediately.
+    Internal function to seed platform plugins for a tenant.
+    Called after transaction commits to ensure schema tables exist.
     """
-    if not created:
-        return
-
-    # Skip public schema
-    if instance.schema_name == 'public':
-        return
-
-    # Defer the import to avoid circular imports
-    from django.db import connection
     from django_tenants.utils import schema_context
     from schedule.models import PluginTemplate
     from django.utils import timezone
 
-    logger.info(f"Seeding platform plugins for new tenant: {instance.schema_name}")
+    logger.info(f"Seeding platform plugins for new tenant: {tenant_schema_name}")
 
     try:
-        with schema_context(instance.schema_name):
+        with schema_context(tenant_schema_name):
             # Import the plugin definitions from the seed command
             from schedule.management.commands.seed_platform_plugins import get_platform_plugins
 
@@ -62,6 +52,28 @@ def seed_platform_plugins_on_tenant_create(sender, instance, created, **kwargs):
                 )
                 created_count += 1
 
-            logger.info(f"Created {created_count} platform plugins for tenant: {instance.schema_name}")
+            logger.info(f"Created {created_count} platform plugins for tenant: {tenant_schema_name}")
     except Exception as e:
-        logger.error(f"Failed to seed plugins for tenant {instance.schema_name}: {e}")
+        logger.error(f"Failed to seed plugins for tenant {tenant_schema_name}: {e}")
+
+
+@receiver(post_save, sender='core.Tenant')
+def seed_platform_plugins_on_tenant_create(sender, instance, created, **kwargs):
+    """
+    Seed platform plugins when a new tenant is created.
+
+    This ensures new tenants have access to all marketplace plugins immediately.
+    Uses transaction.on_commit() to defer seeding until after the schema is
+    fully created and migrations have run.
+    """
+    if not created:
+        return
+
+    # Skip public schema
+    if instance.schema_name == 'public':
+        return
+
+    # Defer the seeding until after the transaction commits
+    # This ensures the schema and all tables exist before we try to use them
+    schema_name = instance.schema_name
+    transaction.on_commit(lambda: _seed_plugins_for_tenant(schema_name))