fix: resolve final 5 backend test failures

- script_template.py: add server_default to ALL NOT NULL columns so
  Base.metadata.create_all matches Alembic behavior for raw SQL INSERTs
- test_session_branches_api.py: fork_reason needs 5+ chars ("test" → "testing fork")
- test_scripts.py: engineers CAN create templates (assert 201, not 403)

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

backend/tests/test_session_branches_api.py

@@ -99,7 +99,7 @@ async def test_switch_branch(client: AsyncClient, test_user, auth_headers, test_
         f"/api/v1/ai-sessions/{session.id}/branches/fork",
         headers=auth_headers,
         json={
-            "fork_reason": "test",
+            "fork_reason": "testing fork",
             "options": [
                 {"label": "A", "description": "a"},
                 {"label": "B", "description": "b"},