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.

modules/overlays.nix

@@ -43,4 +43,36 @@ final: prev: {
       }
     );
   };
+
+  jellyfin-exporter = prev.buildGoModule rec {
+    pname = "jellyfin-exporter";
+    version = "unstable-2025-03-27";
+    src = prev.fetchFromGitHub {
+      owner = "rebelcore";
+      repo = "jellyfin_exporter";
+      rev = "8e3970cb1bdf3cb21fac099c13072bb7c1b20cf9";
+      hash = "sha256-wDnhepYj1MyLRZlwKfmwf4xiEEL3mgQY6V+7TnBd0MY=";
+    };
+    vendorHash = "sha256-e08u10e/wNapNZSsD/fGVN9ybMHe3sW0yDIOqI8ZcYs=";
+    # upstream tests require a running Jellyfin instance
+    doCheck = false;
+    meta.mainProgram = "jellyfin_exporter";
+  };
+
+  igpu-exporter = prev.buildGoModule rec {
+    pname = "igpu-exporter";
+    version = "unstable-2025-03-27";
+    src = prev.fetchFromGitHub {
+      owner = "mike1808";
+      repo = "igpu-exporter";
+      rev = "db2dace1a895c2b950f6d3ba1a2e46729251d124";
+      hash = "sha256-xWTiu26UzTZIK/6jeda+x6VePUgoWTS0AekejFdgFWs=";
+    };
+    vendorHash = "sha256-oeCSKwDKVwvYQ1fjXXTwQSXNl/upDE3WAAk680vqh3U=";
+    subPackages = [ "cmd" ];
+    postInstall = ''
+      mv $out/bin/cmd $out/bin/igpu-exporter
+    '';
+    meta.mainProgram = "igpu-exporter";
+  };
 }