Fix HLS playback auth + add DNB matching source

Player: hls.js did not send Authorization header for segment requests,
causing 401 errors on all HLS fetches. Fixed via xhrSetup callback.

DNB: Added Deutsche Nationalbibliothek SRU search (mat=ton filter for
audiobooks, MARC21-XML parsing). Extracts title, author, narrator,
publisher, year, series, genres, ISBN-based cover URL.

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

backend/app/services/matcher.py

@@ -24,6 +24,7 @@ from .matching.base import MatchResult
 from .matching.musicbrainz import search_musicbrainz, get_release_details
 from .matching.open_library import search_open_library, get_work_details
 from .matching.google_books import search_google_books
+from .matching.dnb import search_dnb
 
 logger = logging.getLogger(__name__)
 
@@ -165,6 +166,7 @@ _SOURCE_FUNCS = {
     "musicbrainz": (search_musicbrainz, get_release_details),
     "open_library": (search_open_library, get_work_details),
     "google_books": (search_google_books, None),
+    "dnb": (search_dnb, None),
 }
 
 
@@ -260,13 +262,14 @@ async def search_for_item(title: str, author: str | None = None) -> list[dict]:
         except Exception:
             return []
 
-    mb, ol, gb = await asyncio.gather(
+    mb, ol, gb, dnb = await asyncio.gather(
         _search_source(search_musicbrainz(title, author)),
         _search_source(search_open_library(title, author)),
         _search_source(search_google_books(title, author)),
+        _search_source(search_dnb(title, author)),
     )
 
-    for r in mb + ol + gb:
+    for r in mb + ol + gb + dnb:
         results.append({
             "source": r.source,
             "id": r.source_id,