docs(admin): archived items page — the missing-file lifecycle

Fourth admin page. Documents the two-consecutive-scans archive rule,
the history-preserving rationale, ARCHIVE_RETENTION_DAYS (default 90,
0 = manual-purge-only, verified in config.go), and the four actions with
verified semantics: Merge (progress/annotations/collections folded into
the active copy, per-user collisions keep the active copy — mirrors
hash-conflict resolution), Restore (immediate; re-archives if the file
is truly still gone), Delete (item + history, permanent), Purge All
Archived (the clean-slate button, confirm text quoted). Links to
scanning.md for why only library-wide scans archive. Glossary gains
Purge All Archived, Merge, and a scoped generic Delete entry.

docs/developer/ui-controls.md

@@ -240,6 +240,18 @@ label: Dismiss
 where: Admin → Libraries → Processing Issues → warning and info cards only
 does: Closes the issue without re-processing, after a confirm. Error-severity issues cannot be dismissed.
 
+label: Purge All Archived
+where: Admin → Archived Items → page header
+does: Permanently deletes every archived item and its reading history, after a confirm. The per-item alternative is Delete.
+
+label: Merge
+where: Admin → Archived Items → rows with a matching active book
+does: Folds the archived copy's reading data (progress, annotations, collections) into the active copy and removes the archived entry; per-user collisions keep the active copy's data.
+
+label: Delete
+where: Admin → Archived Items rows, and other admin destructive rows
+does: Permanently removes the item and its reading history — no archive, no undo. Every Delete is confirm-guarded and states the scope.
+
 label: Resume rewind
 where: audio player → player settings; profile settings
 does: Rewinds playback a configured few seconds when you resume.