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

A cached image was served until eviction, so a change 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 it with no download or
conversion. New bytes replace it, and older variants are converted
again. A failed check serves the cached image and puts the URL in the
negative cache. Images not checked for 30 days are evicted.

Variant rows keep the source's ETag and Last-Modified, so a variant
whose source was evicted is checked the same way.

A variant file's modification time is its source's check time when it
was made or renewed, so a hit reads no database row.

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)
 	}