Add Nix package, NixOS module, and tests for bambuddy

- packages.bambuddy: FastAPI backend + prebuilt React frontend assembled
  into the app-root layout upstream expects, behind a single `bambuddy`
  launcher (uvicorn --loop asyncio, ffmpeg on PATH, XDG state defaults).
  Pins upstream maziggy/bambuddy v1.2.5.7.
- nixosModules.default: services.bambuddy with host/port/dataDir,
  openFirewall, environment + environmentFile, extraPackages,
  DynamicUser + StateDirectory, systemd hardening, and
  CAP_NET_BIND_SERVICE for the virtual-printer FTPS/RTSP ports.
- checks.bambuddy-import: backend import + admin CLI smoke test.
- checks.bambuddy-service: two-node NixOS VM test covering the health
  endpoint, UI/static assets/manifest, SQLite + logs in the
  StateDirectory, journal startup check, cross-node reachability with
  openFirewall, custom host/port/env node, and loopback-only binding.
- overlay, run app, devShell, README, .gitignore.

tests/service-test.nix

@@ -0,0 +1,92 @@
+{
+  lib,
+  pkgs,
+  bambuddy,
+  nixosModule,
+}:
+# Integration test: boots two NixOS VMs with the bambuddy NixOS module.
+#
+#  * `server` uses the default configuration (port 8000, firewall open) and
+#    checks that the service starts, the API responds, the prebuilt React
+#    frontend is served with its static assets, and persistent state
+#    (SQLite database, logs) lands in the StateDirectory.
+#  * `custom` exercises the module options (custom host/port, extra
+#    environment) and verifies the dashboard is reachable cross-node once
+#    the firewall is opened.
+pkgs.testers.runNixOSTest (
+  { ... }:
+  {
+    name = "bambuddy-service";
+
+    nodes = {
+      server =
+        { pkgs, ... }:
+        {
+          imports = [ nixosModule ];
+          services.bambuddy = {
+            enable = true;
+            openFirewall = true;
+          };
+          environment.systemPackages = [ pkgs.curl ];
+        };
+
+      custom =
+        { pkgs, ... }:
+        {
+          imports = [ nixosModule ];
+          services.bambuddy = {
+            enable = true;
+            host = "127.0.0.1";
+            port = 8123;
+            environment.LOG_LEVEL = "DEBUG";
+          };
+          environment.systemPackages = with pkgs; [
+            curl
+            iproute2
+          ];
+        };
+    };
+
+    testScript = ''
+      server.start()
+      custom.start()
+
+      server.wait_for_unit("bambuddy.service")
+      server.wait_for_open_port(8000)
+
+      with subtest("health endpoint responds"):
+          server.succeed("curl -fsS http://127.0.0.1:8000/health | grep -q healthy")
+
+      with subtest("uvicorn started cleanly"):
+          server.succeed(
+              "journalctl -u bambuddy.service --no-pager | grep -q 'Application startup complete'"
+          )
+
+      with subtest("web UI is served from the packaged static build"):
+          server.succeed("curl -fsS http://127.0.0.1:8000/ | grep -q '<title>Bambuddy</title>'")
+          server.succeed("curl -fsS -o /dev/null -w '%{http_code}' http://127.0.0.1:8000/img/favicon-32x32.png | grep -q '^200$'")
+          server.succeed("curl -fsS -o /dev/null -w '%{http_code}' http://127.0.0.1:8000/fonts/inter-latin.woff2 | grep -q '^200$'")
+          server.succeed("curl -fsS http://127.0.0.1:8000/manifest.json | grep -qi bambuddy")
+
+      with subtest("persistent state lands in the StateDirectory"):
+          server.succeed("test -f /var/lib/bambuddy/bambuddy.db")
+          server.succeed("test -d /var/lib/bambuddy/logs")
+
+      with subtest("dashboard is reachable from another machine (openFirewall)"):
+          custom.succeed("curl -m 10 -fsS http://server:8000/health | grep -q healthy")
+
+      with subtest("custom host/port options apply"):
+          custom.wait_for_unit("bambuddy.service")
+          custom.wait_for_open_port(8123)
+          custom.succeed("curl -fsS http://127.0.0.1:8123/health | grep -q healthy")
+          # host = 127.0.0.1 must not appear on any external address
+          custom.fail("ss -tln | grep -E '0\\.0\\.0\\.0:8123|\\[::\\]:8123|\\*:8123'")
+          server.fail("curl -m 5 -fsS http://custom:8123/health")
+
+      with subtest("extra environment reaches the service"):
+          custom.succeed(
+              "systemctl show bambuddy.service -p Environment | grep -q LOG_LEVEL=DEBUG"
+          )
+    '';
+  }
+)