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/httpfetcher/request_id_internal_test.go

@@ -35,7 +35,7 @@ func TestFetchSendsRequestID(t *testing.T) {
 
 	ctx := context.WithValue(testContext(t), middleware.RequestIDKey, requestID)
 
-	res, err := f.Fetch(ctx, upstreamSource("/image"))
+	res, err := f.Fetch(ctx, upstreamSource("/image"), "", "")
 	if err != nil {
 		t.Fatalf("Fetch() error = %v", err)
 	}