docs(admin): processing issues page — per-library scan-failure ledger

Third admin page. Documents the severity stat cards (errors/warnings/
info, cards only appear when non-zero), the per-issue card fields, and
the three actions with their real semantics: Retry (per-item scanner
re-run — success closes the issue, failure surfaces the real error),
Retry all (sequential queue with done/total progress, deliberately not
a library-wide scan), Dismiss (warnings/info only, confirm-guarded;
errors cannot be dismissed). Links to scanning.md for the retry-vs-
library-scan distinction. Glossary gains Retry, Retry all, Dismiss.

docs/developer/ui-controls.md

@@ -228,6 +228,18 @@ label: Scan this folder
 where: Admin → Libraries → a library's panel → a folder row
 does: Shallow-scans only that folder's subtree — unchanged files are skipped, and items in the library's other folders are never marked missing by a partial scan.
 
+label: Retry
+where: Admin → Libraries → a library's Processing Issues page → an issue card
+does: Re-runs that one item through the scanner (a per-item scan, not a library pass). Success closes the issue; failure shows the real error.
+
+label: Retry all
+where: Admin → Libraries → a library's Processing Issues page → beside the severity cards
+does: Queues every issue on the page and re-runs them one at a time, showing done/total progress. Deliberately not a whole-library scan.
+
+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: Resume rewind
 where: audio player → player settings; profile settings
 does: Rewinds playback a configured few seconds when you resume.