docs(admin): libraries page — the definition surface, with backlinks

Fifth admin page and the section's hub. Documents library creation
(name/description/type — type fixed at creation, verified in the edit
modal), the card's Force Rescan + Manage controls, and the full panel:
folders (add via Browse or manual path, shallow per-folder scan,
removal-unregisters-only semantics verified in DeleteLibraryFolder —
files and items untouched, archive lifecycle governs the rest; podcast
roots refuse hand scans), User Access visibility checkboxes (default-
allow, DB-enforced), the processing-issues link, and Delete Library's
exact destructive scope (records + reading data gone, files on disk
untouched). Ends with the at-a-glance scan table linking scanning.md.
Adds the Libraries backlinks to the processing-issues and archived
pages. Glossary gains Create Library, Manage, Edit Details, Delete
Library, View Archived, Purge Archived Now, Browse, User Access.

docs/admin/library-processing-issues.md

@@ -11,7 +11,8 @@
 Processing Issues is each library's ledger of items the scanner could not
 fully process — a corrupt file, an unsupported format, metadata that
 wouldn't parse. It is per-library: you reach it from that library's panel
-on the Libraries page (the link only appears when the library has issues).
+on the [Libraries](libraries.md) page (the link only appears when the
+library has issues).
 
 ## What's on the page