Add the staff stats dashboard

Backend: GET /api/stats/ (staff only) gathers psutil CPU/memory counters,
nvidia-smi GPU stats, the cached disk numbers and the running/finished
download + match jobs. Root logging now also writes a rotating file
(backend/logs/j621.log) so the dashboard can tail it, and psutil joins the
requirements. The storage payload computation is shared with the existing
storage endpoint.

Frontend: a /stats route + Stats nav entry for staff, polling every 2 s —
per-core CPU bars, memory and swap, GPUs (utilization, VRAM, temperature),
disk with the media/temp breakdown, active jobs with progress bars,
recently finished jobs with summaries, and the log tail with level colours
and an auto-scroll toggle. Section 4 of the roadmap is complete.

backend/config/settings.py

@@ -203,6 +203,31 @@ REST_FRAMEWORK = {
 
 DEFAULT_AUTO_FIELD = "django.db.models.BigAutoField"
 
+LOGS_DIR = BASE_DIR / "logs"
+LOG_FILE = LOGS_DIR / "j621.log"
+try:
+    LOGS_DIR.mkdir(parents=True, exist_ok=True)
+except OSError:  # read-only checkout: keep console logging only
+    pass
+
+_log_handlers = {
+    "console": {
+        "class": "logging.StreamHandler",
+        "formatter": "simple",
+    },
+}
+_root_handlers = ["console"]
+if LOGS_DIR.exists():
+    _log_handlers["file"] = {
+        "class": "logging.handlers.RotatingFileHandler",
+        "filename": str(LOG_FILE),
+        "maxBytes": 5 * 1024 * 1024,
+        "backupCount": 3,
+        "encoding": "utf-8",
+        "formatter": "simple",
+    }
+    _root_handlers.append("file")
+
 LOGGING = {
     "version": 1,
     "disable_existing_loggers": False,
@@ -212,14 +237,9 @@ LOGGING = {
             "style": "{",
         },
     },
-    "handlers": {
-        "console": {
-            "class": "logging.StreamHandler",
-            "formatter": "simple",
-        },
-    },
+    "handlers": _log_handlers,
     "root": {
-        "handlers": ["console"],
+        "handlers": _root_handlers,
         "level": "INFO",
     },
 }