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`

frontend/src/pages/BookDetail.tsx

@@ -4,7 +4,7 @@ import {
   Play, ArrowLeft, RefreshCw, Search, Check,
   Loader2, Trash2, X
 } from 'lucide-react'
-import { getItem, updateItem, triggerMatch, searchMatch, applyMatch, coverUrl } from '../api/items'
+import { getItem, updateItem, triggerMatch, searchMatch, applyMatch, debugMatch, extractCover, coverUrl } from '../api/items'
 import { getMe, createBookmark, deleteBookmark } from '../api/me'
 import { usePlayerStore } from '../store/playerStore'
 import CoverImage from '../components/common/CoverImage'
@@ -21,6 +21,8 @@ export default function BookDetail() {
   const [matchQuery, setMatchQuery] = useState('')
   const [matchLoading, setMatchLoading] = useState(false)
   const [showMatchPanel, setShowMatchPanel] = useState(false)
+  const [debugData, setDebugData] = useState<any>(null)
+  const [debugLoading, setDebugLoading] = useState(false)
   const { play, item: currentItem, currentTime } = usePlayerStore()
 
   useEffect(() => {
@@ -79,6 +81,27 @@ export default function BookDetail() {
     }, 3000)
   }
 
+  const handleDebug = async () => {
+    setDebugLoading(true)
+    try {
+      const data = await debugMatch(title, author || undefined)
+      setDebugData(data)
+    } finally {
+      setDebugLoading(false)
+    }
+  }
+
+  const handleExtractCover = async () => {
+    if (!id) return
+    const res = await extractCover(id)
+    if (res.success) {
+      const updated = await getItem(id)
+      setItem(updated)
+    } else {
+      alert('Kein lokales Cover gefunden')
+    }
+  }
+
   const fmtTime = (s: number) => {
     const h = Math.floor(s / 3600)
     const m = Math.floor((s % 3600) / 60)
@@ -167,10 +190,59 @@ export default function BookDetail() {
               <RefreshCw size={14} className={matchLoading ? 'animate-spin' : ''} />
               Auto-Match
             </button>
+            <button
+              onClick={handleDebug}
+              disabled={debugLoading}
+              className="flex items-center gap-2 bg-card border border-divider px-4 py-2 rounded-lg text-sm text-muted hover:text-ink hover:bg-card/80 disabled:opacity-50 transition-colors"
+              title="Zeigt was die APIs tatsächlich zurückgeben"
+            >
+              {debugLoading ? <Loader2 size={14} className="animate-spin" /> : <Search size={14} />}
+              Debug
+            </button>
+            <button
+              onClick={handleExtractCover}
+              className="flex items-center gap-2 bg-card border border-divider px-4 py-2 rounded-lg text-sm text-muted hover:text-ink hover:bg-card/80 transition-colors"
+              title="Cover aus Ordner-Datei oder MP3-Tag extrahieren"
+            >
+              Cover aus Datei
+            </button>
           </div>
         </div>
       </div>
 
+      {debugData && (
+        <div className="mb-6 bg-surface border border-divider rounded-xl p-4">
+          <div className="flex items-center justify-between mb-3">
+            <h3 className="text-ink" style={{ fontSize: '13px', fontWeight: 600 }}>API-Debug</h3>
+            <button onClick={() => setDebugData(null)} className="text-muted hover:text-ink">
+              <X size={14} />
+            </button>
+          </div>
+          <p className="text-muted mb-2" style={{ fontSize: '12px' }}>
+            Such-Titel (bereinigt): <span className="font-mono text-ink">{debugData.normalized?.search_title}</span>
+            {debugData.normalized?.series && <> · Serie: <span className="text-primary">{debugData.normalized.series}</span></>}
+            {debugData.normalized?.episode && <> · Folge: <span className="text-primary">{debugData.normalized.episode}</span></>}
+          </p>
+          {debugData.sources?.map((s: any) => (
+            <div key={s.source} className="mb-3 border-t border-divider pt-3">
+              <p className="text-ink mb-1" style={{ fontSize: '12px', fontWeight: 600 }}>
+                {s.source} — {s.ok ? `${s.count} Treffer` : <span className="text-red-400">Fehler: {s.error}</span>}
+              </p>
+              {s.ok && s.results?.slice(0, 3).map((r: any, i: number) => (
+                <div key={i} className="text-muted ml-2 mb-1" style={{ fontSize: '11px' }}>
+                  <p className="text-ink">{r.title} {r.author && `— ${r.author}`} {r.year && `(${r.year})`}</p>
+                  {r.narrator && <p>Sprecher: {r.narrator}</p>}
+                  {r.publisher && <p>Verlag: {r.publisher}</p>}
+                  {r.series && <p>Serie: {r.series} {r.series_sequence && `#${r.series_sequence}`}</p>}
+                  {r.cover_url && <p className="break-all">Cover: <span className="font-mono">{r.cover_url}</span></p>}
+                  <p className="opacity-50">conf={r.confidence} id={r.source_id}</p>
+                </div>
+              ))}
+            </div>
+          ))}
+        </div>
+      )}
+
       {/* Description */}
       {meta.description && (
         <div className="mb-6">