Persist video predictions in a unified media cache and add a cache browser

[govza]

Summary

• Cache TTL: clamp HTTP max-age to a 30–90 day window and ignore Expires.
• Telemetry: log warn/error as plain text so chrome://extensions errors are readable.
• Video prediction cache: persist Frame Sample predictions per video key, seed the verdict timeline on attach and answer covered samples locally (within 0.25 s). Rows keep forced visibility and model id and are cleared on strictness change.
• MediaCacheDatabase: replaces ImageDatabase with one DB holding imagePredictions and videoPredictions, both indexed by [hostname+timestamp]. Legacy ImageDatabase/VideoDatabase are deleted on install/update, so the cache starts empty.
• Options → Cache tab: images/videos grouped by site, paged per site via the compound index, per-item/per-site/all delete. Previews load only on click, so the page makes no network requests on its own.

Test plan

• pnpm test:unit (652 passing), typecheck, lint
• Load extension, browse a few sites, confirm Options → Cache lists sites and items
• Open a site with >25 items, page through
• Click a preview; remove item / clear site / clear all
• Update over an existing install and confirm old databases are gone