Fix matching: add missing subtitle field, proper error logging, match-all endpoint

- MatchResult was missing subtitle field, causing AttributeError in
  _apply_match that silently killed every background match task
- Wrap _apply_match in try/except with exc_info logging so failures
  are visible in docker compose logs backend
- New POST /api/libraries/:id/match-all endpoint to trigger matching
  for all unlocked items (useful for items scanned before the fix)
- Admin UI: Match button per library next to the Scan button

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

backend/app/routers/libraries.py

@@ -219,3 +219,33 @@ async def scan_library(
     background_tasks.add_task(scan_library_task, library_id, job.id)
 
     return {"id": job.id, "type": "scan", "libraryId": library_id}
+
+
+@router.post("/{library_id}/match-all")
+async def match_all_items(
+    library_id: str,
+    background_tasks: BackgroundTasks,
+    admin: User = Depends(require_admin),
+    db: AsyncSession = Depends(get_db),
+):
+    """Startet Matching für alle nicht-gematchten Items der Library."""
+    result = await db.execute(select(Library).where(Library.id == library_id))
+    if not result.scalar_one_or_none():
+        raise HTTPException(status_code=404, detail="Library not found")
+
+    items_result = await db.execute(
+        select(LibraryItem).where(
+            LibraryItem.library_id == library_id,
+            LibraryItem.match_locked == False,
+        )
+    )
+    items = items_result.scalars().all()
+    item_ids = [i.id for i in items]
+
+    async def _run_all():
+        from ..services.matcher import match_audiobook
+        for iid in item_ids:
+            await match_audiobook(iid)
+
+    background_tasks.add_task(_run_all)
+    return {"queued": len(item_ids), "libraryId": library_id}