grafana: replace custom metric collectors with community exporters

Replace three custom Prometheus textfile collector scripts with
dedicated community-maintained exporters:

- jellyfin-collector.nix (25 LoC shell) -> rebelcore/jellyfin_exporter
  Metric: jellyfin_active_streams -> count(jellyfin_now_playing_state)
  Bonus: per-session labels (user, title, device, codec info)

- qbittorrent-collector.nix (40 LoC shell) -> anriha/qbittorrent-metrics-exporter
  Metric: qbittorrent_{download,upload}_bytes_per_second -> qbit_{dl,up}speed
  Bonus: per-torrent metrics with category/tag aggregation

- intel-gpu-collector.nix + .py (130 LoC Python) -> mike1808/igpu-exporter
  Metric: intel_gpu_engine_busy_percent -> igpu_engines_busy_percent
  Bonus: persistent daemon vs oneshot timer, no streaming JSON parser

All three run as persistent daemons scraped by Prometheus, replacing
the textfile-collector pattern of systemd timers writing .prom files.
Dashboard PromQL queries updated to match new metric names.

services/grafana/prometheus.nix

@@ -71,6 +71,24 @@ in
           { targets = [ "127.0.0.1:${toString service_configs.ports.private.llama_cpp.port}" ]; }
         ];
       }
+      {
+        job_name = "jellyfin";
+        static_configs = [
+          { targets = [ "127.0.0.1:${toString service_configs.ports.private.jellyfin_exporter.port}" ]; }
+        ];
+      }
+      {
+        job_name = "qbittorrent";
+        static_configs = [
+          { targets = [ "127.0.0.1:${toString service_configs.ports.private.qbittorrent_exporter.port}" ]; }
+        ];
+      }
+      {
+        job_name = "igpu";
+        static_configs = [
+          { targets = [ "127.0.0.1:${toString service_configs.ports.private.igpu_exporter.port}" ]; }
+        ];
+      }
     ];
   };