feat(db): implement PostgreSQL monitoring and maintenance features

Added PostgreSQL monitoring and maintenance capabilities to the API, including new endpoints for instance-level metrics, maintenance operations, and job scheduling. Updated the HTTP API to support PostgreSQL monitoring routes and integrated a background scheduler for metrics collection. Enhanced the CLI with database commands for maintenance tasks. Updated documentation to reflect these changes.

internal/httpapi/routes_postgres_test.go

@@ -0,0 +1,21 @@
+package httpapi
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+func TestPostgresOverviewMemoryBackend503(t *testing.T) {
+	srv, err := New(Options{SeedDemo: true})
+	if err != nil {
+		t.Fatal(err)
+	}
+	req := httptest.NewRequest(http.MethodGet, "/v1/monitoring/postgres/overview", nil)
+	req.Header.Set("Authorization", "Bearer dev")
+	rec := httptest.NewRecorder()
+	srv.Handler().ServeHTTP(rec, req)
+	if rec.Code != http.StatusServiceUnavailable {
+		t.Fatalf("status=%d body=%s", rec.Code, rec.Body.String())
+	}
+}