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>

backend/app/routers/matching.py

@@ -52,6 +52,63 @@ async def search_match(
     return {"results": results}
 
 
+@router.get("/match/debug")
+async def debug_match(
+    title: str,
+    author: str | None = None,
+    current_user: User = Depends(get_current_user),
+):
+    """Debug-Endpoint: gibt rohe Ergebnisse aller Such-Quellen zurück.
+    Aufruf direkt aus Browser: /api/items/match/debug?title=Foo&author=Bar
+    """
+    from ..services.matching.musicbrainz import search_musicbrainz
+    from ..services.matching.open_library import search_open_library
+    from ..services.matching.google_books import search_google_books
+    from ..services.matching.dnb import search_dnb
+    from ..services.matcher import _build_search_title, detect_series
+
+    series, episode = detect_series(title)
+    search_title = _build_search_title(title)
+    if series and episode:
+        search_title = f"{series} {episode}"
+
+    logger.info(f"DEBUG: title={title!r} → search={search_title!r} series={series!r} episode={episode!r}")
+
+    async def _try(name, coro):
+        try:
+            r = await coro
+            return {
+                "source": name,
+                "ok": True,
+                "count": len(r),
+                "results": [
+                    {
+                        "title": x.title, "author": x.author, "narrator": x.narrator,
+                        "publisher": x.publisher, "year": x.publish_year,
+                        "series": x.series, "series_sequence": x.series_sequence,
+                        "cover_url": x.cover_url, "language": x.language,
+                        "genres": x.genres, "description": (x.description or "")[:200],
+                        "confidence": x.confidence, "source_id": x.source_id,
+                    } for x in r
+                ],
+            }
+        except Exception as e:
+            return {"source": name, "ok": False, "error": f"{type(e).__name__}: {e}"}
+
+    results = await asyncio.gather(
+        _try("musicbrainz", search_musicbrainz(search_title, author)),
+        _try("open_library", search_open_library(search_title, author)),
+        _try("google_books", search_google_books(search_title, author)),
+        _try("dnb", search_dnb(search_title, author)),
+    )
+
+    return {
+        "input": {"title": title, "author": author},
+        "normalized": {"search_title": search_title, "series": series, "episode": episode},
+        "sources": results,
+    }
+
+
 @router.post("/{item_id}/match/apply")
 async def apply_match(
     item_id: str,