Make matching debuggable + fix metadata search blockers

DNB rewrite:
- Multiple query strategies with fallback (title+author+mat=ton →
  title+author → title+mat=ton → title-only → fulltext). Returns on
  first hit. Most German audiobooks aren't tagged mat=ton in DNB,
  which was killing all searches.
- Strip CQL wildcard chars (?, *, <, >, =, /, quotes) from search
  terms. The "???" in "Die drei ???" was breaking the CQL parser.
- Log HTTP status, body snippet on non-200, and numberOfRecords on
  every query so log shows exactly what DNB returned.
- Parse SRU diagnostic elements (DNB error messages buried in XML).
- Convert author/narrator from "Lastname, Firstname" to
  "Firstname Lastname" for consistency with other sources.

Matcher:
- Split series patterns: WITH_EPISODE (need digit) and SERIES_ONLY
  (just the series name). "Die drei ??? und der Fluch des Rubins"
  now properly detects "Die drei ???" as series even without folge#.
- New _build_search_title: removes ??? sequences, trailing parens,
  collapses whitespace, before sending to APIs.
- Manual search also passes through normalization. Logs source +
  hit count per query.

Debug endpoint:
- GET /api/items/match/debug?title=...&author=... returns raw results
  from all 4 sources with status, error messages, and full metadata.
- "Debug" button added in BookDetail — shows what each API actually
  returns inline, so the user can see if it's a search problem,
  parse problem, or threshold problem.
- "Cover aus Datei" button — triggers local cover extraction
  (folder.jpg or embedded artwork) on demand.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/api/items.ts

@@ -27,4 +27,10 @@ export const applyMatch = (id: string, match: object) =>
 export const triggerMatch = (id: string) =>
   api.post(`/api/items/${id}/match`).then((r) => r.data)
 
+export const debugMatch = (title: string, author?: string) =>
+  api.get(`/api/items/match/debug`, { params: { title, author } }).then((r) => r.data)
+
+export const extractCover = (id: string) =>
+  api.post(`/api/items/${id}/extract-cover`).then((r) => r.data)
+
 export const coverUrl = (id: string) => `/api/items/${id}/cover`