Refuse an unparseable exp with 400; log swallowed cache errors (closes #72)

An exp that was not a whole number, or empty, was ignored, so a URL for
a host that needs a signature got 401 as if it had no exp. It is now a
400 naming exp and the value, on every host; only an exp missing from
the URL is unchanged.

A failed variant .meta write, source metadata JSON write, Stats count
query, stats counter update, negative cache write or expired negative
cache delete was discarded without a trace. Each is now logged at warn
with the path or key and the error, and stays non-fatal, with tests for
those that can be made to fail. VariantStorage takes the cache's logger.

Model: opus-5-5

internal/imgcache/storage_internal_test.go

@@ -4,8 +4,10 @@ import (
 	"bytes"
 	"errors"
 	"io"
+	"log/slog"
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 )
 
@@ -404,3 +406,35 @@ func TestCacheKey(t *testing.T) {
 		t.Error("CacheKey() produced same key for different quality")
 	}
 }
+
+// TestVariantStorage_StoreLogsFailedMetaWrite verifies that a .meta write
+// that fails is logged at warn and the store still succeeds.
+func TestVariantStorage_StoreLogsFailedMetaWrite(t *testing.T) {
+	t.Parallel()
+
+	var logBuf bytes.Buffer
+
+	storage, err := NewVariantStorage(
+		t.TempDir(), slog.New(slog.NewJSONHandler(&logBuf, nil)))
+	if err != nil {
+		t.Fatalf("NewVariantStorage() error = %v", err)
+	}
+
+	key := CacheKey(&ImageRequest{SourceHost: testHostCDN, SourcePath: testPathCat})
+
+	// A directory where the .meta file goes makes the .meta write fail.
+	err = os.MkdirAll(storage.keyToPath(key)+".meta", StorageDirPerm)
+	if err != nil {
+		t.Fatalf("failed to create directory: %v", err)
+	}
+
+	_, err = storage.Store(key, bytes.NewReader([]byte("variant data")), "image/webp")
+	if err != nil {
+		t.Fatalf("Store() error = %v, want nil", err)
+	}
+
+	want := `"level":"WARN","msg":"failed to write variant metadata sidecar"`
+	if !strings.Contains(logBuf.String(), want) {
+		t.Errorf("log missing %s; got %q", want, logBuf.String())
+	}
+}