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)

.dockerignore

@@ -0,0 +1,30 @@
+# dependencies
+node_modules/
+backend/__pycache__/
+backend/**/__pycache__/
+backend/*.py[cod]
+.venv/
+venv/
+
+# data & runtime
+data/
+*.log
+
+# git
+.git/
+.gitignore
+.gitattributes
+
+# IDE
+.idea/
+.vscode/
+*.swp
+*.swo
+
+# OS
+.DS_Store
+Thumbs.db
+
+# env
+.env
+.env.*