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/routers/logs.py

@@ -0,0 +1,57 @@
+import os
+from fastapi import APIRouter, Depends, HTTPException, Query
+from ..dependencies import require_admin
+from ..models.user import User
+from ..config import get_settings
+
+router = APIRouter(prefix="/api/logs", tags=["logs"])
+
+_LOG_FILES = {
+    "app": "app.log",
+    "matching": "matching.log",
+}
+
+
+@router.get("/{log_name}")
+async def get_log(
+    log_name: str,
+    lines: int = Query(300, ge=10, le=5000),
+    _admin: User = Depends(require_admin),
+):
+    if log_name not in _LOG_FILES:
+        raise HTTPException(status_code=404, detail="Unbekannte Log-Datei")
+
+    settings = get_settings()
+    path = os.path.join(settings.log_dir, _LOG_FILES[log_name])
+
+    if not os.path.exists(path):
+        return {"lines": [], "total": 0, "showing": 0, "exists": False, "path": path}
+
+    with open(path, "r", encoding="utf-8", errors="replace") as f:
+        all_lines = f.readlines()
+
+    last = all_lines[-lines:]
+    return {
+        "lines": [l.rstrip("\n") for l in last],
+        "total": len(all_lines),
+        "showing": len(last),
+        "exists": True,
+        "path": path,
+    }
+
+
+@router.delete("/{log_name}")
+async def clear_log(
+    log_name: str,
+    _admin: User = Depends(require_admin),
+):
+    if log_name not in _LOG_FILES:
+        raise HTTPException(status_code=404, detail="Unbekannte Log-Datei")
+
+    settings = get_settings()
+    path = os.path.join(settings.log_dir, _LOG_FILES[log_name])
+
+    if os.path.exists(path):
+        open(path, "w").close()
+
+    return {"success": True}