Add observability stack: Alloy, Tempo, and OTEL support

- Add NixOS modules for Alloy and Tempo with default configs
- Update Grafana datasource config for Prometheus, Loki, Tempo
- Add Prometheus remote_write for Alloy
- Implement OTEL metrics/tracing/logging in arrtrix (Go)
- Enable Alloy and Tempo in ulmo system config

packages/arrtrix/pkg/observability/otel_test.go

@@ -0,0 +1,54 @@
+package observability
+
+import "testing"
+
+func TestConfigDefaults(t *testing.T) {
+	var cfg Config
+	cfg.ApplyDefaults()
+
+	if cfg.ServiceName != "arrtrix" {
+		t.Fatalf("expected default service name arrtrix, got %q", cfg.ServiceName)
+	}
+	if cfg.ResourceAttributes == nil {
+		t.Fatal("expected resource attributes map to be initialized")
+	}
+	if cfg.Enabled() {
+		t.Fatal("expected observability to be disabled by default")
+	}
+}
+
+func TestParseEndpointSupportsURLAndBareHost(t *testing.T) {
+	tests := []struct {
+		name      string
+		input     string
+		wantRaw   string
+		insecure  bool
+		wantError bool
+	}{
+		{name: "https url", input: "https://otel.example:4317", wantRaw: "https://otel.example:4317"},
+		{name: "http url", input: "http://127.0.0.1:4317", wantRaw: "http://127.0.0.1:4317", insecure: true},
+		{name: "bare host", input: "collector:4317", wantRaw: "http://collector:4317", insecure: true},
+		{name: "invalid", input: "://bad", wantError: true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := parseEndpoint(tt.input)
+			if tt.wantError {
+				if err == nil {
+					t.Fatal("expected error")
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("parseEndpoint returned error: %v", err)
+			}
+			if got.raw != tt.wantRaw {
+				t.Fatalf("expected raw endpoint %q, got %q", tt.wantRaw, got.raw)
+			}
+			if got.insecure != tt.insecure {
+				t.Fatalf("expected insecure=%t, got %t", tt.insecure, got.insecure)
+			}
+		})
+	}
+}