Remove the unused cache lifetime setting (closes #69)

CacheConfig.CacheTTL was set to DefaultCacheTTL (24 hours) and never
read, so cached images never expired. Remove the field, the constant
and the line that set it. Cached source and transformed images stay
until eviction removes them to keep the cache within cache_max_bytes.

A new test checks that a source and a transformed image cached two
days ago are still served from the cache without a new upstream fetch.

Existing tests lose only the CacheTTL line of their cache config; no
test's checks change.

Model: opus-5-5

internal/imgcache/service_internal_test.go

@@ -2,7 +2,9 @@ package imgcache
 
 import (
 	"context"
+	"encoding/json"
 	"io"
+	"os"
 	"testing"
 	"time"
 
@@ -438,8 +440,9 @@ func TestService_Get_ServesSourceAndVariantCachedTwoDaysAgo(t *testing.T) {
 	t.Parallel()
 
 	svc, fixtures := SetupTestService(t)
+	variant := photoVariant(fixtures, 85, FitCover)
 
-	resp, err := svc.Get(t.Context(), photoVariant(fixtures, 85, FitCover))
+	resp, err := svc.Get(t.Context(), variant)
 	if err != nil {
 		t.Fatalf("Get() first request error = %v", err)
 	}
@@ -462,7 +465,31 @@ func TestService_Get_ServesSourceAndVariantCachedTwoDaysAgo(t *testing.T) {
 		}
 	}
 
-	resp, err = svc.Get(t.Context(), photoVariant(fixtures, 85, FitCover))
+	// The variant's creation time is also its file's modification time
+	// and the created_at in its .meta file
+	twoDaysAgo := time.Now().AddDate(0, 0, -2)
+	variantPath := svc.cache.variants.keyToPath(CacheKey(variant))
+
+	err = os.Chtimes(variantPath, twoDaysAgo, twoDaysAgo)
+	if err != nil {
+		t.Fatalf("setting the variant file's modification time back: %v", err)
+	}
+
+	meta, err := json.Marshal(VariantMeta{
+		ContentType: resp.ContentType,
+		Size:        resp.ContentLength,
+		CreatedAt:   twoDaysAgo.Unix(),
+	})
+	if err != nil {
+		t.Fatalf("encoding .meta file: %v", err)
+	}
+
+	err = os.WriteFile(variantPath+".meta", meta, StorageFilePerm)
+	if err != nil {
+		t.Fatalf("writing .meta file: %v", err)
+	}
+
+	resp, err = svc.Get(t.Context(), variant)
 	if err != nil {
 		t.Fatalf("Get() of the cached variant error = %v", err)
 	}