feat: add audit log table and integration with admin/tree endpoints

Creates AuditLog model with JSONB details column for tracking admin
actions. Integrates log_audit() helper into admin endpoints (role
change, team admin toggle, deactivate, activate) and tree delete.
IP address column reserved for future Railway proxy header support.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/models/__init__.py

@@ -10,6 +10,7 @@ from .folder import UserFolder, user_folder_trees
 from .step_category import StepCategory
 from .step_library import StepLibrary, StepRating, StepUsageLog
 from .refresh_token import RefreshToken
+from .audit_log import AuditLog
 
 __all__ = [
     "User",
@@ -28,4 +29,5 @@ __all__ = [
     "StepRating",
     "StepUsageLog",
     "RefreshToken",
+    "AuditLog",
 ]