Check cached sources with upstream after the cache TTL (closes #69)

A cached source and its variants were served until eviction, so a
changed image at the same URL was never picked up.

After 24 hours a request checks the source with upstream, sending its
stored ETag and Last-Modified. A 304 renews the source with no download
or conversion. New bytes replace the source, and older variants are
converted again. A failed check serves the cached image and puts the
URL in the negative cache. This includes the full fetch made for a
variant whose source was evicted. Images not checked for 30 days are
evicted.

A variant file's modification time is its source's check time, so a
hit still makes no database call.

Model: opus-5-5

internal/handlers/referer_blocklist_internal_test.go

@@ -36,10 +36,11 @@ type countingFetcher struct {
 // Fetch counts the fetch and passes it on.
 func (f *countingFetcher) Fetch(
 	ctx context.Context, source imagerequest.Source,
+	ifNoneMatch, ifModifiedSince string,
 ) (*httpfetcher.FetchResult, error) {
 	f.fetches.Add(1)
 
-	return f.Fetcher.Fetch(ctx, source)
+	return f.Fetcher.Fetch(ctx, source, ifNoneMatch, ifModifiedSince)
 }
 
 // newRefererRoutes returns both image routes of a Handlers whose