fix: complete remaining 8 optimization items

- HTTP connection pooling: UpstreamClient & WebsiteClient reuse httpx.Client
- Deduplicate decimal_string into shared app/utils/number.py
- Split scheduler transaction: snapshot write → webhook/website sync in separate sessions
- Remove hardcoded 170.106.100.210 migration from database.py
- Reset consecutive_failures on upstream update
- Healthcheck: install curl, replace python -c with curl -f
- Add .dockerignore to reduce build context
- Frontend: add axios-retry with exponential backoff (5xx/network errors only)

Dockerfile

@@ -25,6 +25,7 @@ RUN apt-get update \
       libfreetype6 libgbm1 libglib2.0-0t64 libgtk-3-0t64 libnspr4 libnss3 \
       libpango-1.0-0 libx11-6 libx11-xcb1 libxcb1 libxcomposite1 \
       libxdamage1 libxext6 libxfixes3 libxrandr2 libxshmfence1 xvfb \
+      curl \
     && rm -rf /var/lib/apt/lists/*
 RUN playwright install chromium