Stabilize existence-cache feature.

Also, make the logging a bit quieter.

src/existence.go

@@ -54,14 +54,12 @@ type ExistenceCache interface {
 }
 
 func CreateExistenceCache(ctx context.Context) (ExistenceCache, error) {
-	if config.Feature("existence-cache") {
-		switch config.Storage.Type {
-		case "s3":
-			maxAge := time.Duration(config.Storage.S3.SiteCache.MaxAge)
-			return createBloomExistenceCache(ctx, maxAge)
-		default:
-			// not needed
-		}
+	switch config.Storage.Type {
+	case "s3":
+		maxAge := time.Duration(config.Storage.S3.SiteCache.MaxAge)
+		return createBloomExistenceCache(ctx, maxAge)
+	default:
+		// not needed
 	}
 
 	return &dummyExistenceCache{}, nil
@@ -156,7 +154,7 @@ func (c *bloomExistenceCache) CheckSite(ctx context.Context, site string) (resul
 	}
 	c.filterMu.Unlock()
 
-	logc.Printf(ctx, "existence: site %s: %s", site, result)
+	logc.Debugf(ctx, "existence: site %s: %s", site, result)
 	return
 }
 
@@ -174,7 +172,7 @@ func (c *bloomExistenceCache) CheckDomain(ctx context.Context, domain string) (r
 	}
 	c.filterMu.Unlock()
 
-	logc.Printf(ctx, "existence: domain %s: %s", domain, result)
+	logc.Debugf(ctx, "existence: domain %s: %s", domain, result)
 	return
 }
 

src/log.go

@@ -31,6 +31,14 @@ func (l slogWithCtx) log(ctx context.Context, level slog.Level, msg string) {
 	logger.Handler().Handle(ctx, record)
 }
 
+func (l slogWithCtx) Debugln(ctx context.Context, v ...any) {
+	l.log(ctx, slog.LevelDebug, fmt.Sprintln(v...))
+}
+
+func (l slogWithCtx) Debugf(ctx context.Context, format string, v ...any) {
+	l.log(ctx, slog.LevelDebug, fmt.Sprintf(format, v...))
+}
+
 func (l slogWithCtx) Printf(ctx context.Context, format string, v ...any) {
 	l.log(ctx, slog.LevelInfo, fmt.Sprintf(format, v...))
 }

src/main.go

@@ -44,11 +44,11 @@ func configureFeatures(ctx context.Context) (err error) {
 	for _, feature := range config.Features {
 		switch feature {
 		// Work-in-progress features:
-		case "preview", "existence-cache":
+		case "preview":
 		// Permanently unstable features:
 		case "codeberg-pages-compat", "relaxed-idna":
 		// Stabilized features:
-		case "archive-site", "audit", "compress", "patch", "serve-h2c":
+		case "archive-site", "audit", "compress", "patch", "serve-h2c", "existence-cache":
 			logc.Printf(ctx, "feature %s has been stabilized", feature)
 		// Removed or renamed features:
 		case "h2c", "sentry-telemetry-buffer", "site-existence-cache", "domain-existence-cache":