fix(ci): python3-venv 自動安裝，確保 venv 建立不失敗

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

.gitea/workflows/cd.yaml

@@ -78,7 +78,8 @@ jobs:
 
           if [ ! -d "$VENV" ] || [ "$(cat $HASH_FILE 2>/dev/null)" != "$CURRENT_HASH" ]; then
             echo "📦 deps 已變更，重建 venv..."
-            python3 -m venv $VENV
+            # 2026-04-05 Claude Code: 確保 python3-venv 已安裝（runner image 可能未內建）
+            python3 -m venv $VENV 2>/dev/null || (apt-get install -y -q python3-venv && python3 -m venv $VENV)
             source $VENV/bin/activate
             pip install -q uv
             cd apps/api && uv pip install -q -e ".[dev]" && cd -