docs: update documentation for blocklist & search non-admin eligibility

- CHANGELOG: document button availability changes (all admin downloads, non-admin eligibility)
- README: update blocklist-search endpoint description with non-admin conditions
- ARCHITECTURE.md: update Authorisation Matrix, Download object table (add canBlocklist, addedOn fields), and blocklist-search API reference

CHANGELOG.md

@@ -24,6 +24,9 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 - History records are now deduplicated server-side before being sent to the client — only the most relevant record per content item per instance is returned.
 - Import-issue badge tooltip now uses the themed `var(--surface)` / `var(--text-primary)` / `var(--border)` CSS variables, matching the episode and toggle tooltip style.
 - Poller now stores `_instanceKey` on Sonarr/Radarr queue records in the cache, enabling the backend to look up API credentials for blocklist operations without an additional configuration lookup.
+- **Blocklist & Search button** — now available on all admin downloads (not just those with import issues), and also available to non-admin users when: import issues are present, OR (for qBittorrent torrents only) the download is more than 1 hour old AND has less than 100% availability.
+- **Download object** — added `canBlocklist` boolean field to indicate whether the current user can blocklist a given download.
+- **qBittorrent torrent data** — added `addedOn` timestamp field to enable age-based blocklist eligibility checks.
 
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