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/service.go

@@ -322,7 +322,12 @@ func (s *Service) fetchAndProcess(
 		// Store negative cache for certain errors
 		if isNegativeCacheable(err) {
 			statusCode := extractStatusCode(err)
-			_ = s.cache.StoreNegative(ctx, req, statusCode, err.Error())
+
+			storeErr := s.cache.StoreNegative(ctx, req, statusCode, err.Error())
+			if storeErr != nil {
+				s.log.Warn("failed to store negative cache entry",
+					"host", req.SourceHost, "path", req.SourcePath, "error", storeErr)
+			}
 		}
 
 		return nil, fmt.Errorf("upstream fetch failed: %w", err)