Serve SPA with CDN-friendly caching and add ETag-based caching for API responses

[zhenghaoz]

Motivation

• Improve caching behavior for the single-page app to be CDN-friendly and reduce origin load by setting explicit cache headers.
• Reduce API response bandwidth and latency by adding ETag support so clients can receive 304 Not Modified when appropriate.

Description

• Added make_spa_response() which serves index.html with Cache-Control: public, max-age=0, s-maxage=60, stale-while-revalidate=30 for CDN-friendly caching.
• Added make_api_cached_response(payload) which computes a SHA256 ETag for the JSON payload, returns 304 when If-None-Match matches, and sets ETag, Cache-Control, and Vary headers.
• Replaced direct app.send_static_file("index.html") calls in "/", "/login", "/privacy", and the 404 handler with make_spa_response().
• Updated cache-hit and response paths in "/api/trending" and "/api/hackernews" to use make_api_cached_response() instead of manually creating responses.

Testing

• Ran the project's automated test suite (unit and integration tests) and the modified endpoints passed with no test failures.

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Codex Task