Add logging system: app.log + matching.log with admin viewer

- Backend: RotatingFileHandler for app.log (all) and matching.log (matcher/matching services)
- New GET/DELETE /api/logs/{app|matching} endpoints (admin-only)
- matcher.py: improved cover-download logging (URL, bytes, HTTP errors, missing cover URL)
- Frontend: Logs tab in Admin panel with log switcher, line count selector, color-coded ERROR/WARNING lines, clear button

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

backend/app/services/matcher.py

@@ -85,6 +85,7 @@ async def _download_cover(url: str, item_id: str) -> str | None:
     if ".png" in url:
         ext = ".png"
     dest = os.path.join(settings.covers_dir, f"{item_id}{ext}")
+    logger.info(f"Cover-Download: {url}")
     try:
         async with httpx.AsyncClient(timeout=20, follow_redirects=True) as client:
             r = await client.get(url)
@@ -92,9 +93,12 @@ async def _download_cover(url: str, item_id: str) -> str | None:
                 os.makedirs(settings.covers_dir, exist_ok=True)
                 with open(dest, "wb") as f:
                     f.write(r.content)
+                logger.info(f"Cover gespeichert: {dest} ({len(r.content)} Bytes)")
                 return dest
+            else:
+                logger.warning(f"Cover-Download HTTP {r.status_code}: {url}")
     except Exception as e:
-        logger.warning(f"Cover-Download fehlgeschlagen ({url}): {e}")
+        logger.warning(f"Cover-Download Fehler ({url}): {e}")
     return None
 
 
@@ -133,6 +137,8 @@ async def _apply_match(db: AsyncSession, item: LibraryItem, result: MatchResult,
         cover_path = await _download_cover(result.cover_url, item.id)
         if cover_path:
             item.cover_path = cover_path
+    elif not result.cover_url:
+        logger.info(f"Kein Cover-URL in Match-Ergebnis ({result.source}: {result.source_id})")
 
     # Kapitel aus MusicBrainz-Tracklisting
     if result.chapters: