@JsonProperty makes the computed getter part of every Document response
Jackson produces, so any DTO returning a Document automatically carries
the thumbnail URL without per-controller plumbing. The accompanying
comment warns future readers that the cache-buster is load-bearing
for the endpoint's `immutable` cache header (CWE-525) (#309).
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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