Fix Alembic migration head conflict

Update down_revision to chain after 018e16dad6aa (BIGINT id column)
instead of the shared parent efbbe5d5862f, resolving the multiple
heads error.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: dpage

web/migrations/versions/add_tools_ai_perm_.py

@@ -10,7 +10,7 @@
 """Add tools_ai permission to existing roles
 
 Revision ID: add_tools_ai_perm
-Revises: efbbe5d5862f
+Revises: 018e16dad6aa
 Create Date: 2025-12-01
 
 """
@@ -19,7 +19,7 @@
 
 # revision identifiers, used by Alembic.
 revision = 'add_tools_ai_perm'
-down_revision = 'efbbe5d5862f'
+down_revision = '018e16dad6aa'
 branch_labels = None
 depends_on = None