ETag handling for conditional GETs (cheap caching)

ETags are a low-effort way to cut bandwidth and CPU when clients poll for resources that rarely change. The server computes an ETag for the representation (often a version, content hash, or updated_at value) and compares it to If-None-Match. If they match, we return 304 Not Modified with no body. The detail that matters is consistency: your ETag must change whenever the response body would change, or caches lie. I like using stable version strings (like a row version or sha256) rather than timestamps that can drift. I also set Cache-Control to a short max-age for CDN friendliness. In practice this is a huge win for endpoints like profile pages, settings payloads, and feature-flag documents.