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)

backend/app/routers/upstreams.py

@@ -121,6 +121,8 @@ def update_upstream(
         data["base_url"] = data["base_url"].rstrip("/")
     for k, v in data.items():
         setattr(u, k, v)
+    # Reset failure counter on any update — the user may have fixed the issue
+    u.consecutive_failures = 0
     u.updated_at = datetime.now(timezone.utc)
     db.commit()
     db.refresh(u)