Gitea #1: prevents blank image flashes while photos load. Blurhash is
computed client-side at upload time (canvas downsampling + the
blurhash package) for posts, milestones, and stories, stored alongside
each PostImage/Story row, and decoded into a smooth color placeholder
via the new BlurImage component that cross-fades to the loaded photo.
Wired into every content-photo surface: feed cards, post detail zoom,
reposts, milestone cards, story viewer, explore/search grids,
notification thumbnails, profile grid, and the admin posts/reports
panels. Pet avatars and ad creatives intentionally excluded — separate
content pipelines with disproportionate effort for the payoff.