refactor(homepage/icon): check service health before fetching icons and add retry logic

The icon fetching logic now checks if the target service is healthy before
attempting to fetch icons. If the health monitor reports an unhealthy status,
the function returns HTTP 503 Service Unavailable instead of proceeding.

Additionally, the icon cache lookup now includes infinite retry logic with a
15-second backoff interval, improving resilience during transient service
outages. Previously, failed lookups would not be retried.

The `route` interface was extended with a `HealthMonitor()` method to support
the health check functionality.

internal/homepage/icons/fetch/fetch.go

@@ -6,6 +6,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"math"
 	"net/http"
 	"net/url"
 	"slices"
@@ -163,6 +164,9 @@ func FindIcon(ctx context.Context, r route, uri string, variant icons.Variant) (
 		}
 	}
 	if r, ok := r.(httpRoute); ok {
+		if mon := r.HealthMonitor(); mon != nil && !mon.Status().Good() {
+			return FetchResultWithErrorf(http.StatusServiceUnavailable, "service unavailable")
+		}
 		// fallback to parse html
 		return findIconSlowCached(context.WithValue(ctx, "route", contextValue{r: r, uri: uri}), r.Key())
 	}
@@ -172,7 +176,7 @@ func FindIcon(ctx context.Context, r route, uri string, variant icons.Variant) (
 var findIconSlowCached = cache.NewKeyFunc(func(ctx context.Context, key string) (Result, error) {
 	v := ctx.Value("route").(contextValue)
 	return findIconSlow(ctx, v.r, v.uri, nil)
-}).WithMaxEntries(200).Build() // no retries, no ttl
+}).WithMaxEntries(200).WithRetriesConstantBackoff(math.MaxInt, 15*time.Second).Build() // infinite retries, 15 seconds interval
 
 func findIconSlow(ctx context.Context, r httpRoute, uri string, stack []string) (Result, error) {
 	select {

internal/homepage/icons/fetch/route.go

@@ -4,6 +4,7 @@ import (
 	"net/http"
 
 	nettypes "github.com/yusing/godoxy/internal/net/types"
+	"github.com/yusing/godoxy/internal/types"
 	"github.com/yusing/goutils/pool"
 )
 
@@ -12,6 +13,7 @@ type route interface {
 	ProviderName() string
 	References() []string
 	TargetURL() *nettypes.URL
+	HealthMonitor() types.HealthMonitor
 }
 
 type httpRoute interface {