fix(ci): B5 整合測試改用 /opt/api-venv/bin/pytest (venv 內的 pytest)

python3.11 -m pytest 找不到模組 (exitcode 1)
改用持久化 venv 路徑，與 Run API Tests step 共用同一 venv

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

.gitea/workflows/cd.yaml

@@ -134,7 +134,7 @@ jobs:
             psql -U awoooi -d awoooi_test \
             -f /dev/stdin < tests/integration/setup_test_schema.sql
           TEST_DATABASE_URL="postgresql+asyncpg://awoooi:awoooi_test_2026@127.0.0.1:15432/awoooi_test" \
-            python3.11 -m pytest tests/integration/test_b5_core_flows.py -v --tb=short
+            /opt/api-venv/bin/pytest tests/integration/test_b5_core_flows.py -v --tb=short
           TEST_EXIT=$?
           docker compose -f docker-compose.test.yml down -v 2>/dev/null || true
           exit $TEST_EXIT