Add CORS support and response caching to aggregate endpoints

- Introduced CORS configuration options in config.example.yaml, allowing specification of allowed origins for cross-origin requests.
- Enhanced the aggregate handler to support response caching with a configurable TTL, improving performance for repeated requests.
- Updated the aggregate API to return a structured response indicating whether any upstream requests failed, enhancing error handling and response clarity.
- Modified documentation in AGGREGATE.md and README.md to reflect the new CORS and caching features.
- Added tests to validate the new functionality in the aggregate handler.

internal/aggregate/fleet.go

@@ -0,0 +1,67 @@
+package aggregate
+
+import (
+	"context"
+	"net/http"
+	"sort"
+	"sync"
+
+	"github.com/telemt/telemt-api/internal/config"
+)
+
+// FetchFleetStatus probes GET health and GET system/info for each alias in parallel.
+func FetchFleetStatus(ctx context.Context, client *http.Client, parsed *config.Parsed, aliases []string) FleetStatusData {
+	if len(aliases) == 0 {
+		return FleetStatusData{}
+	}
+	rows := make([]FleetServerStatus, len(aliases))
+	var wg sync.WaitGroup
+	for i, alias := range aliases {
+		i, alias := i, alias
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			rows[i] = probeFleetServer(ctx, client, parsed, alias)
+		}()
+	}
+	wg.Wait()
+	sort.Slice(rows, func(i, j int) bool { return rows[i].Alias < rows[j].Alias })
+	allOK, failed := 0, 0
+	for _, r := range rows {
+		if r.OK {
+			allOK++
+		} else {
+			failed++
+		}
+	}
+	return FleetStatusData{
+		Servers:       rows,
+		ServersTotal:  len(rows),
+		ServersAllOK:  allOK,
+		ServersFailed: failed,
+	}
+}
+
+func probeFleetServer(ctx context.Context, client *http.Client, parsed *config.Parsed, alias string) FleetServerStatus {
+	h, hm := FetchTelemtGET[HealthData](ctx, client, parsed, alias, "health")
+	s, sm := FetchTelemtGET[SystemInfoData](ctx, client, parsed, alias, "system/info")
+	row := FleetServerStatus{Alias: alias}
+	row.HealthOK = hm.OK
+	row.HealthHTTPStatus = hm.HTTPStatus
+	row.HealthLatencyMs = hm.LatencyMs
+	row.HealthError = hm.Error
+	row.HealthRevision = hm.Revision
+	if hm.OK {
+		row.Health = &h
+	}
+	row.SystemInfoOK = sm.OK
+	row.SystemInfoHTTPStatus = sm.HTTPStatus
+	row.SystemInfoLatencyMs = sm.LatencyMs
+	row.SystemInfoError = sm.Error
+	row.SystemInfoRevision = sm.Revision
+	if sm.OK {
+		row.SystemInfo = &s
+	}
+	row.OK = hm.OK && sm.OK
+	return row
+}