Refresh should return 304 / not reship the whole poll payload

Problem/Motivation

With polling enabled, the refresh endpoint returns the whole poll payload every interval, almost always unchanged. Measured: 172,673 bytes and ~1.8 s every 30 s per placement. The current set does not address the poll payload size.

Steps to reproduce

• Enable polling and watch the refresh request on a 200+ comment thread.
• Observe the full payload is re-sent each interval with no change.

Proposed resolution

• Compute an ETag from the comment list tag checksum plus the max comment changed timestamp.
• Return 304 when the client's ETag matches, with the same freshness guarantees.

Remaining tasks

• Verify the common unchanged poll becomes a 304.

User interface changes

None, beyond less network traffic.

API changes

Adds ETag support to the refresh route.

Data model changes

None anticipated.

AI-Generated: Yes (Used Opus 5 and DeepSeek Flash to review and discover these issues)